drm/i915: Extract skl_plane_ctl()

Pull the code to calculate the SKL plane control register value into
a separate function. Allows us to pre-compute it in the future.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170317211808.14693-2-ville.syrjala@linux.intel.com


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