drm/cirrus: Remove unecessary NULL check in gem_free

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: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>