drm/amdgpu: set preferred_domain independent of fallback handling

When GEM needs to fallback to GTT for VRAM BOs we still want the
preferred domain to be untouched so that the BO has a cance to move back
to VRAM in the future.

Signed-off-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>