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