drm/i915/gvt: Set initial PORT_CLK_SEL vreg for BDW

drm/i915/gvt: Set initial PORT_CLK_SEL vreg for BDW

On BDW, when host physical screen and guest virtual screen aren't on
the same DDI port, guest i915 driver prints the following error and
stop running.
[    6.775873] BUG: unable to handle kernel NULL pointer dereference
at 0000000000000068
[    6.775928] IP: intel_ddi_clock_get+0x81/0x430 [i915]
[    6.776206] Call Trace:
[    6.776233]  ? vgpu_read32+0x4f/0x100 [i915]
[    6.776264]  intel_ddi_get_config+0x11c/0x230 [i915]
[    6.776298]  intel_modeset_setup_hw_state+0x313/0xd40 [i915]
[    6.776334]  intel_modeset_init+0xe49/0x18d0 [i915]
[    6.776368]  ? vgpu_write32+0x53/0x100 [i915]
[    6.776731]  ? intel_i2c_reset+0x42/0x50 [i915]
[    6.777085]  ? intel_setup_gmbus+0x32a/0x350 [i915]
[    6.777427]  i915_driver_load+0xabc/0x14d0 [i915]
[    6.777768]  i915_pci_probe+0x4f/0x70 [i915]

The null pointer is guest intel_crtc_state->shared_dpll which is
setted in haswell_get_ddi_pll(). When guest and host screen are
on different DDI port, host driver won't set PORT_CLK_SET(guest_port),
so haswell_get_ddi_pll() will return null and don't set
pipe_config->shared_dpll, once the following program refernce this
structure, it will print the above error.

This patch set the initial val of guest PORT_CLK_SEL(guest_port) to
LCPLL_810. And guest i915 driver will reset this value according to
guest screen mode.

Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>