You can enable the CRTC and without adding the plane to the state and
it will succeed. This should be prevented in the crtc check instead of
the plane check, because the plane check may never run for atomic
enable, but the crtc check always will.

This is based on a similar check in vmwgfx.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: David Airlie <airlied@linux.ie>
Link: http://patchwork.freedesktop.org/patch/msgid/20170712081344.25495-2-maarten.lankhorst@linux.intel.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>