drm/i915: Use primary plane->state for overlay ckey setup

Extract the primary plane pixel format via plane state when setting up
the overlay colorkey.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1481131693-27993-10-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>