drm/amdgpu: initial validate the prime BOs into the CPU domain

Just set the GTT domain as mandatory, so that the BO is validated into
it on first use. This allows us to setup the sg table later on.

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