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

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.com
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>