drm/i915: Move SKL hw stride calculation into a helper

We repeat the SKL stride register value calculations a several places.
Move it into a small helper function.

v2: Rebase due to drm_plane_state src/dst rects

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470821001-25272-4-git-send-email-ville.syrjala@linux.intel.com