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: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170317211808.14693-2-ville.syrjala@linux.intel.comReviewed-by: NChris Wilson <chris@chris-wilson.co.uk>