drm: Use drm_mode_is_420_only() instead of open coding it

Replace the open coded drm_mode_is_420_only() with the real thing.

No functional changes.

Cc: Werner Sembach <wse@tuxedocomputers.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210504102742.7005-1-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/drm_modes.c

@@ -1176,16 +1176,11 @@ enum drm_mode_status
 drm_mode_validate_ycbcr420(const struct drm_display_mode *mode,
 			   struct drm_connector *connector)
 {
-	u8 vic = drm_match_cea_mode(mode);
-	enum drm_mode_status status = MODE_OK;
-	struct drm_hdmi_info *hdmi = &connector->display_info.hdmi;
-
-	if (test_bit(vic, hdmi->y420_vdb_modes)) {
-		if (!connector->ycbcr_420_allowed)
-			status = MODE_NO_420;
-	}
+	if (!connector->ycbcr_420_allowed &&
+	    drm_mode_is_420_only(&connector->display_info, mode))
+		return MODE_NO_420;
 
-	return status;
+	return MODE_OK;
 }
 EXPORT_SYMBOL(drm_mode_validate_ycbcr420);