drm/radeon: avoid UVD corruptions on AGP cards

commit 4f66c59922cbcda14c9e103e6c7f4ee616360d43 upstream.

Putting everything into VRAM seems to help.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>