drm/gm12u320: Use drmm_add_final_kfree

With this we can drop the final kfree from the release function.
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-18-daniel.vetter@ffwll.ch

drivers/gpu/drm/tiny/gm12u320.c

@@ -19,6 +19,7 @@
 #include <drm/drm_gem_shmem_helper.h>
 #include <drm/drm_gem_framebuffer_helper.h>
 #include <drm/drm_ioctl.h>
+#include <drm/drm_managed.h>
 #include <drm/drm_modeset_helper_vtables.h>
 #include <drm/drm_probe_helper.h>
 #include <drm/drm_simple_kms_helper.h>
@@ -637,7 +638,6 @@ static void gm12u320_driver_release(struct drm_device *dev)
 	gm12u320_usb_free(gm12u320);
 	drm_mode_config_cleanup(dev);
 	drm_dev_fini(dev);
-	kfree(gm12u320);
 }
 
 DEFINE_DRM_GEM_FOPS(gm12u320_fops);
@@ -692,6 +692,7 @@ static int gm12u320_usb_probe(struct usb_interface *interface,
 		return ret;
 	}
 	dev->dev_private = gm12u320;
+	drmm_add_final_kfree(dev, gm12u320);
 
 	drm_mode_config_init(dev);
 	dev->mode_config.min_width = GM12U320_USER_WIDTH;