drm/qxl: Use the drm_driver.dumb_destroy default

drm_gem_dumb_destroy() is the drm_driver.dumb_destroy default,
so no need to set it.

Cc: Dave Airlie <airlied@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1502034068-51384-10-git-send-email-noralf@tronnes.org

drivers/gpu/drm/qxl/qxl_drv.c

@@ -263,7 +263,6 @@ static struct drm_driver qxl_driver = {
 
 	.dumb_create = qxl_mode_dumb_create,
 	.dumb_map_offset = qxl_mode_dumb_mmap,
-	.dumb_destroy = drm_gem_dumb_destroy,
 #if defined(CONFIG_DEBUG_FS)
 	.debugfs_init = qxl_debugfs_init,
 #endif