drm: Wrap direct calls to driver->gem_free_object from CMA

Since the introduction of (struct_mutex) lockless GEM bo freeing, there
are a pair of driver vfuncs for freeing the GEM bo, of which the driver
may choose to only implement driver->gem_object_free_unlocked (and so
avoid taking the struct_mutex along the free path). However, the CMA GEM
helpers were still calling driver->gem_free_object directly, now NULL,
and promptly dying on the fancy new lockless drivers. Oops.

Robert Foss bisected this to b82caafcf2303 (drm/vc4: Use lockless gem BO
free callback) on his vc4 device, but that just serves as an enabler for
9f0ba539d13ae (drm/gem: support BO freeing without dev->struct_mutex).

Reported-by: Robert Foss <robert.foss@collabora.com>
Fixes: 9f0ba539d13ae (drm/gem: support BO freeing without dev->struct_mutex)
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Robert Foss <robert.foss@collabora.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Eric Anholt <eric@anholt.net>
Cc: Alex Deucher <alexdeucher@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: stable@vger.kernel.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Robert Foss <robert.foss@collabora.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_fb_cma_helper.c

@@ -445,7 +445,7 @@ err_cma_destroy:
 err_fb_info_destroy:
 	drm_fb_helper_release_fbi(helper);
 err_gem_free_object:
-	dev->driver->gem_free_object(&obj->base);
+	drm_gem_object_unreference_unlocked(&obj->base);
 	return ret;
 }
 EXPORT_SYMBOL(drm_fbdev_cma_create_with_funcs);

drivers/gpu/drm/drm_gem_cma_helper.c

@@ -121,7 +121,7 @@ struct drm_gem_cma_object *drm_gem_cma_create(struct drm_device *drm,
 	return cma_obj;
 
 error:
-	drm->driver->gem_free_object(&cma_obj->base);
+	drm_gem_object_unreference_unlocked(&cma_obj->base);
 	return ERR_PTR(ret);
 }
 EXPORT_SYMBOL_GPL(drm_gem_cma_create);
@@ -162,18 +162,12 @@ drm_gem_cma_create_with_handle(struct drm_file *file_priv,
 	 * and handle has the id what user can see.
 	 */
 	ret = drm_gem_handle_create(file_priv, gem_obj, handle);
-	if (ret)
-		goto err_handle_create;
-
 	/* drop reference from allocate - handle holds it now. */
 	drm_gem_object_unreference_unlocked(gem_obj);
+	if (ret)
+		return ERR_PTR(ret);
 
 	return cma_obj;
-
-err_handle_create:
-	drm->driver->gem_free_object(gem_obj);
-
-	return ERR_PTR(ret);
 }
 
 /**