drm/radeon: Drop radeon_gem_object_(un)pin.

Only radeon_gem_object_unpin was used anymore, in only one place.

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>