drm/i915/skl: intel_format_to_fourcc() doesn't work for SKL planes

We will have a skl_ version shortly!

Reviewed-By: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>