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

commit 4ca5a6cba53e13b8fd153b0762b4128fab6a3cfb upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>