drm/atomic: Don't open-code CRTC state destroy

One failure path in crtc_helper had an open-coded CRTC state destroy
which didn't actually call through to the driver's specified state
destroy. Fix that.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>