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

v2: Split the pre-ilk vs. ilk+ unification to a separate patch (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170323192712.30682-2-ville.syrjala@linux.intel.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>