drm/i915: Take a runtime pm wakeref for atomic commits

Before we start prepping the system for an atomic modeset, wake the
device up. We then keep track of this wakeref until we complete the
atomic commit, so we hold keep the device awake for all potential HW
access, and do not allow the device to sleep with a pending modeset.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110771


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190528075354.22341-1-chris@chris-wilson.co.uk