drm/radeon/kms: fix a regression on r7xx AGP due to the HDP flush fix

drm/radeon/kms: fix a regression on r7xx AGP due to the HDP flush fix

commit 87cbf8f2c5d1b1fc4642c3dc0bb6efc587479603 upstream.

commit: 812d046915f48236657f02c06d7dc47140e9ceda
drm/radeon/kms/r7xx: add workaround for hw issue with HDP flush
breaks on AGP boards since there is no VRAM gart table.

This patch fixes the issue by creating a VRAM scratch page so that
can be used on both AGP and PCIE.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=29834

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>