drm/i915/gen9: Initialize intel_state->active_crtcs during WM sanitization (v2)

intel_state->active_crtcs is usually only initialized when doing a
modeset.  During our first atomic commit after boot, we're effectively
faking a modeset to sanitize the DDB/wm setup, so ensure that this field
gets initialized before use.

v2:
 - Don't clobber active_crtcs if our first commit really is a modeset
   (Maarten)
 - Grab connection_mutex when faking a modeset during sanitization
   (Maarten)

Reported-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Tested-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466196140-16336-2-git-send-email-matthew.d.roper@intel.com
Cc: stable@vger.kernel.org #v4.7+
(cherry picked from commit 1b54a880b250acc226b13cea221b90aa1b3e37dd)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>