drm/prime: warn for non-empty handle lookup list during drm file release

drm_gem_release should release all handles connected to the drm file and
so should also release the prime lookup entries of these handles. So
just WARN if this isn't the case.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>