drm: Use the driver's gem_object_free function from CMA helpers.

VC4 wraps the CMA objects in its own structures, so it needs to do its
own teardown (waiting for GPU to finish, updating bo_stats tracking).
The other CMA drivers are using drm_gem_cma_free_object as their
gem_free_object, so this should be a no-op for them.

Signed-off-by: Eric Anholt <eric@anholt.net>