drm: Load EDID: Explain better how to write your own EDID firmware

A description was lacking how to write an EDID firmware file that
corresponds to a given X11 setting.

Signed-off-by: Carsten Emde <C.Emde@osadl.org>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>