drm/i915: Introduce skl_plane_ddb_iter

Collect a bit of the stuff used during the plane ddb
allocation into a struct we can pass around.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220118092354.11631-5-ville.syrjala@linux.intel.com


Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>