Having the EDID override debugfs directly update the EDID property is
problematic. The update is partial only. The driver has no way of
knowing it's been updated. Mode list is not updated. It's an
inconsistent state.

Detach debugfs EDID override from the property update completely. Only
set and reset a separate override EDID copy from debugfs, and have it
take effect only at detect (via EDID read). The copy is at
connector->edid_override, protected by connector->edid_override_mutex.

This also brings override EDID closer to firmware EDID in behaviour.

Add validation of the override EDID which we completely lacked.

Note that IGT already forces a detect whenever tests update the override
EDID.

v2: Add locking (Ville)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/4c875f8e06c4499f498fcf876e1233cbb155ec8a.1666614699.git.jani.nikula@intel.com