drm/i915/gen9: Cache plane data rates in CRTC state

This will be important when we start calculating CRTC data rates for
in-flight CRTC states since it will allow us to calculate the total data
rate without needing to grab the plane state for any planes that aren't
updated by the transaction.

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/1463061971-19638-4-git-send-email-matthew.d.roper@intel.com