drm/edid: fix CEA extension byte #3 parsing

Only an EDID CEA extension has byte #3, while the CTA DisplayID Data
Block does not. Don't interpret bogus data for color formats.

For most displays it's probably an unlikely scenario you'd have a CTA
DisplayID Data Block without a CEA extension, but they do exist.

Fixes: e28ad54 ("drm/edid: parse CEA blocks embedded in DisplayID")
Cc: <stable@vger.kernel.org>
Cc: Shawn C Lee <shawn.c.lee@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220323100438.1757295-1-jani.nikula@intel.com

drivers/gpu/drm/drm_edid.c

@@ -5188,10 +5188,14 @@ static void drm_parse_cea_ext(struct drm_connector *connector,
 
 	/* The existence of a CEA block should imply RGB support */
 	info->color_formats = DRM_COLOR_FORMAT_RGB444;
-	if (edid_ext[3] & EDID_CEA_YCRCB444)
-		info->color_formats |= DRM_COLOR_FORMAT_YCBCR444;
-	if (edid_ext[3] & EDID_CEA_YCRCB422)
-		info->color_formats |= DRM_COLOR_FORMAT_YCBCR422;
+
+	/* CTA DisplayID Data Block does not have byte #3 */
+	if (edid_ext[0] == CEA_EXT) {
+		if (edid_ext[3] & EDID_CEA_YCRCB444)
+			info->color_formats |= DRM_COLOR_FORMAT_YCBCR444;
+		if (edid_ext[3] & EDID_CEA_YCRCB422)
+			info->color_formats |= DRM_COLOR_FORMAT_YCBCR422;
+	}
 
 	if (cea_db_offsets(edid_ext, &start, &end))
 		return;