drm/i915: Only update state on crtc's that are part of the atomic state.

This is probably hard to hit right now because in most cases all
atomic locks are taken, but after conversion to atomic this will make
it more likely to corrupt the crtc->config pointer, resulting in hard
to find bugs.

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