drm/i915: Get rid of crtc_state->fb_changed

We had this as an optimization to not do a plane update, but we killed
it off because there are so many reasons we may have to do a plane
update or fastset that it's best to just assume everything changed.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190920114235.22411-6-maarten.lankhorst@linux.intel.com


Reviewed-by: Matt Roper <matthew.d.roper@intel.com>