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

drivers/gpu/drm/drm_connector.c

@@ -960,6 +960,11 @@ static const struct drm_prop_enum_list dp_colorspaces[] = {
  * 	drm_connector_update_edid_property(), usually after having parsed
  * 	the EDID using drm_add_edid_modes(). Userspace cannot change this
  * 	property.
+ *
+ * 	User-space should not parse the EDID to obtain information exposed via
+ * 	other KMS properties (because the kernel might apply limits, quirks or
+ * 	fixups to the EDID). For instance, user-space should not try to parse
+ * 	mode lists from the EDID.
  * DPMS:
  * 	Legacy property for setting the power state of the connector. For atomic
  * 	drivers this is only provided for backwards compatibility with existing