drm: document that user-space should avoid parsing EDIDs

User-space should avoid parsing EDIDs for metadata already exposed via
other KMS interfaces and properties. For instance, user-space should not
try to extract a list of modes from the EDID: the kernel might mutate
the mode list (because of link capabilities or quirks for instance).

Other metadata not exposed by KMS can be parsed by user-space. This
includes for instance monitor identification (make/model/serial) and
supported color-spaces.

v2: add short explanation why user-space shouldn't do this (Brian)

Signed-off-by: Simon Ser <contact@emersion.fr>
Suggested-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Brian Starkey <brian.starkey@arm.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/V_APW3gKRhljvcmT28tGV3JkP7qW9Z7h45I-s2wiJvYhaaveCpYpg3tztZPsZVV2KV1NC7rUx08IUUgCJXzdRrWCsEGB0czq4ZozpdyVFLs=@emersion.fr