Share the code to compute the primary plane control register value
between the i9xx and ilk codepaths as the differences are minimal.
Actually there are no differences between g4x and ilk, so the
current split doesn't really make any sense.

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/20170323192712.30682-3-ville.syrjala@linux.intel.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>