drm/atomic: add connector mask to drm_crtc_state.

It can be useful to iterate over connectors without grabbing
connection_mutex. It can also be used to see how many connectors
are on a crtc without iterating over the list.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1451908400-25147-4-git-send-email-maarten.lankhorst@linux.intel.com


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>