drm/i915: Refactor CURPOS calculation

Move the CURPOS calculations to seprate function. This will allow
sharing the code between the 845/865 vs. others codepaths when we
otherwise split them apart.

v2: Don't pass intel_plane as it's not needed

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/20170327185546.2977-6-ville.syrjala@linux.intel.com