drm/radeon/uvd: lower msg&fb buffer requirements on UVD3

Starting with UVD3 message and feedback buffers have their
own 256MB segment, so no need to force them into VRAM any more.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>