The ->gem_free_object never gets called with a NULL pointer, the check
is redundant. Also checking after the upcast allows compilers to elide
it anyway.

Spotted by coverity.
Reviewed-by: NDavid Herrmann <dh.herrmann@gmail.com>
Reviewed-by: NIan Romanick <ian.d.romanick@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>