drm/radeon: always set GEM function pointer

Make sure to always set the GEM function pointer even for in kernel
allocations. This fixes a NULL pointer deref caused by switching to GEM
references.

Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: fd69ef05 ("drm/radeon: use GEM references instead of TTMs")
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 01b64bc0)