drm/i915/skl: Remove unnecessary local variables in skl_plane_ctl*()

Ville noticed in another patch we we didn't need them at all, so remove
them. It's worth saying that it makes no difference to code generated as
gcc is clever enough to optimize it out.

v2: Remove 'break' after 'return' in switches (Ville)

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>