drm/radeon: fix "force the UVD DPB into VRAM as well"

The DPB must be in VRAM, but not in the first segment.

Signed-off-by: Christian König <christian.koenig@amd.com>
Tested-by: Arthur Marsh <arthur.marsh@internode.on.net>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>