drm/atomic: fix potential null ptr on plane enable

When a plane is being enabled, plane->crtc has not been set yet.  Use
plane->state->crtc.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>