drm/radeon: avoid UVD corruption on AGP cards using GPU gart

If the user has forced the driver to use the internal GPU gart
rather than AGP on an AGP card, force the buffers to vram
as well.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>
Cc: stable@vger.kernel.org