drm/i915: Save user requested plane coordinates only on success

If the setplane operation fails, we shouldn't save the user's requested
plane coordinates. Since we adjust the coordinates during the clipping
process, make a copy of the originals, and once the operation has
succeeded save them for later reuse when the plane gets re-enabled.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>