drm crtc: use drm_mode_destroy instead of kfree in drm_mode_remove

Modes are created using drm_mode_create which does a
drm_mode_object_get, so use drm_mode_destroy in drm_mode_remove
which does a drm_mode_object_put.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>