drm/edid: Don't include ext block csum in DispID size

The EDID extension block checksum byte is not part of the
actual DispID data, so don't use it in validate_displayid().
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200313162054.16009-8-ville.syrjala@linux.intel.comReviewed-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/drm_edid.c

@@ -3222,7 +3222,8 @@ static u8 *drm_find_displayid_extension(const struct edid *edid,
 	if (!displayid)
 		return NULL;
 
-	*length = EDID_LENGTH;
+	/* EDID extensions block checksum isn't for us */
+	*length = EDID_LENGTH - 1;
 	*idx = 1;
 
 	ret = validate_displayid(displayid, *length, *idx);