drm/i915: Pre-compute plane control register value

Computing the plane control register value is branchy so moving it out
from the plane commit hook seems prudent. Let's pre-compute it during
the atomic check phase and store the result in the plane state.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170323192712.30682-4-ville.syrjala@linux.intel.com