drm/i915/skl: Use a more idomatic early return

We can coalesce the WARN() condition with the WARN() itself and, as we
are returning early, we can de-intent the rest of the function.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>