drm: bochs: Don't remove uninitialized fbdev framebuffer

drm: bochs: Don't remove uninitialized fbdev framebuffer

In the same spirit of the fix for QXL in commit 861078381ba5 ("drm: qxl:
Don't alloc fbdev if emulation is not supported"), prevent the Oops in
the unbind path of Bochs if fbdev emulation is disabled.

[  112.176009] Oops: 0002 [#1] SMP
[  112.176009] Modules linked in: bochs_drm
[  112.176009] CPU: 0 PID: 3002 Comm: bash Not tainted 4.11.0-rc1+ #111
[  112.176009] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.9.3-20161025_171302-gandalf 04/01/2014
[  112.176009] task: ffff8800743bbac0 task.stack: ffffc90000b5c000
[  112.176009] RIP: 0010:mutex_lock+0x18/0x30
[  112.176009] RSP: 0018:ffffc90000b5fc78 EFLAGS: 00010246
[  112.176009] RAX: 0000000000000000 RBX: 0000000000000260 RCX: 0000000000000000
[  112.176009] RDX: ffff8800743bbac0 RSI: ffff8800787176e0 RDI: 0000000000000260
[  112.176009] RBP: ffffc90000b5fc80 R08: ffffffff00000000 R09: 00000000ffffffff
[  112.176009] R10: ffff88007b463650 R11: 0000000000000000 R12: 0000000000000260
[  112.176009] R13: ffff8800787176e0 R14: ffffffffa0003068 R15: 0000000000000060
[  112.176009] FS:  00007f20564c7b40(0000) GS:ffff88007ce00000(0000) knlGS:0000000000000000
[  112.176009] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  112.176009] CR2: 0000000000000260 CR3: 000000006b89c000 CR4: 00000000000006f0
[  112.176009] Call Trace:
[  112.176009]  drm_mode_object_unregister+0x1e/0x50
[  112.176009]  drm_framebuffer_unregister_private+0x15/0x20
[  112.176009]  bochs_fbdev_fini+0x57/0x70 [bochs_drm]
[  112.176009]  bochs_unload+0x16/0x50 [bochs_drm]
[  112.176009]  drm_dev_unregister+0x37/0xd0
[  112.176009]  drm_put_dev+0x31/0x60
[  112.176009]  bochs_pci_remove+0x10/0x20 [bochs_drm]
[  112.176009]  pci_device_remove+0x34/0xb0
[  112.176009]  device_release_driver_internal+0x150/0x200
[  112.176009]  device_release_driver+0xd/0x10
[  112.176009]  unbind_store+0x108/0x150
[  112.176009]  drv_attr_store+0x20/0x30
[  112.176009]  sysfs_kf_write+0x32/0x40
[  112.176009]  kernfs_fop_write+0x10b/0x190
[  112.176009]  __vfs_write+0x23/0x120
[  112.176009]  ? security_file_permission+0x36/0xb0
[  112.176009]  ? rw_verify_area+0x49/0xb0
[  112.176009]  vfs_write+0xb0/0x190
[  112.176009]  SyS_write+0x41/0xa0
[  112.176009]  entry_SYSCALL_64_fastpath+0x1a/0xa9
[  112.176009] RIP: 0033:0x7f2055bd5620
[  112.176009] RSP: 002b:00007ffed2f487d8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[  112.176009] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f2055bd5620
[  112.176009] RDX: 000000000000000d RSI: 0000000000ee0008 RDI: 0000000000000001
[  112.176009] RBP: 0000000000000001 R08: 00007f2055e94760 R09: 00007f20564c7b40
[  112.176009] R10: 0000000000000073 R11: 0000000000000246 R12: 0000000000000000
[  112.176009] R13: 00007ffed2f48d70 R14: 0000000000000000 R15: 0000000000000000
[  112.176009] Code: 00 00 00 55 be 02 00 00 00 48 89 e5 e8 62 fb ff ff 5d c3 55 48 89 e5 53 48 89 fb e8 53 e9 ff ff 65 48 8b 14 25 40 c4 00 00 31 c0 <f0> 48 0f b1 13 48 85 c0 74 08 48 89 df e8c6 ff ff ff 5b 5d c3
[  112.176009] RIP: mutex_lock+0x18/0x30 RSP: ffffc90000b5fc78
[  112.176009] CR2: 0000000000000260
[  112.205622] ---[ end trace 76189cd7a9bdd155 ]---

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170317181409.4183-1-krisman@collabora.co.uk
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>