drm/i915/skl: Add a new line before return

We usually use a new line before those kind of return statements. Also
the various skl_plane_ctl*() functions weren't consistent.

Cc: Chandra Konduru <chandra.konduru@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>