drm/i915/gen9: fail the modeset instead of WARNing on unsupported config

Now that this code is part of the compute stage we can return -EINVAL
to prevent the modeset instead of giving a WARN and trying anyway.

v2:
 - Fix typo (Paul Menzel).
 - Add MISSING_CASE() (Ville, Maarten).

Reported-by: Lyude <cpaul@redhat.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474578035-424-10-git-send-email-paulo.r.zanoni@intel.com