drm/i915: create struct intel_plane_state

This new struct will be the storage of src and dst coordinates
between the check and commit stages of a plane update.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>