drm/edid: Yank a helpful comment about EST modes from xf86EdidModes.c

I got very confused when I tried to compare the EST modes with the spec.
Bring over a comment from xf86EdidModes.c that actually describes some
of history where these things came from.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>