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

commit: 812d0469
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>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>