drm/i915/skl: handle all pixel formats in skylake_update_primary_plane()

skylake_update_primary_plane() did not handle all pixel formats returned
by skl_format_to_fourcc(). Handle alpha similar to skl_update_plane().

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89052
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>