drm/i915: Introduce intel_crtc_update_sarea_pos()

Refactor the code that stores the panning x/y position into the sarea.

This also changes the code so that it won't mistakenly update
sareaB_x/y for pipe >= C.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>