drm/i915/skl: Replace BUG() by MISSING_CASE() in skl_plane_ctl_format()

Let's be consistent with the others skl_plane_ctl_*() functions and use
a MISSING_CASE(). Not only that, but it's a rude to BUG() the whole
machine here.

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