drm/i915: Update connector_mask during readout, v2.

The connector_mask may be used any time during the non-atomic
.crtc_disable which is called before the full atomic state is
set up and needs to be accurate for that reason.

Changes since v1:
- Update connector_mask in readout_hw_state and add a comment.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/568D1C55.8010001@linux.intel.comSigned-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>