drm/edid: Document why we don't bounds check the DispID CEA block start/end

After much head scratching I managed to convince myself that
for_each_displayid_db() has already done the bounds checks for
the DispID CEA data block. Which is why we don't need to repeat
them in cea_db_offsets(). To avoid having to go through that
pain again in the future add a comment which explains this fact.

Cc: Andres Rodriguez <andresx7@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200124200231.10517-5-ville.syrjala@linux.intel.comAcked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>

drivers/gpu/drm/drm_edid.c

@@ -4017,6 +4017,10 @@ cea_db_offsets(const u8 *cea, int *start, int *end)
 	 *   no non-DTD data.
 	 */
 	if (cea[0] == DATA_BLOCK_CTA) {
+		/*
+		 * for_each_displayid_db() has already verified
+		 * that these stay within expected bounds.
+		 */
 		*start = 3;
 		*end = *start + cea[2];
 	} else if (cea[0] == CEA_EXT) {