drm/i915: Track whether an atomic transaction changes the active CRTC's

For the purposes of DDB re-allocation we need to know whether a
transaction changes the list of CRTC's that are active.  While
state->modeset could be used for this purpose, that would be slightly
too aggressive since it would lead us to re-allocate the DDB when a
CRTC's mode changes, but not its final active state.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463061971-19638-7-git-send-email-matthew.d.roper@intel.com