drm: Do not accept garbage mode aspect ratio flags

Don't let userspace feed us any old garbage in the mode aspect ratio
flags.

Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190620142639.17518-3-ville.syrjala@linux.intel.comReviewed-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/drm_modes.c

@@ -1977,9 +1977,11 @@ int drm_mode_convert_umode(struct drm_device *dev,
 	case DRM_MODE_FLAG_PIC_AR_256_135:
 		out->picture_aspect_ratio = HDMI_PICTURE_ASPECT_256_135;
 		break;
-	default:
+	case DRM_MODE_FLAG_PIC_AR_NONE:
 		out->picture_aspect_ratio = HDMI_PICTURE_ASPECT_NONE;
 		break;
+	default:
+		return -EINVAL;
 	}
 
 	out->status = drm_mode_validate_driver(dev, out);