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

drivers/gpu/drm/i915/intel_pm.c

@@ -3580,11 +3580,12 @@ static int skl_compute_plane_wm(const struct drm_i915_private *dev_priv,
 		case 2:
 			y_min_scanlines = 8;
 			break;
-		default:
-			WARN(1, "Unsupported pixel depth for rotation");
 		case 4:
 			y_min_scanlines = 4;
 			break;
+		default:
+			MISSING_CASE(cpp);
+			return -EINVAL;
 		}
 	} else {
 		y_min_scanlines = 4;