drm/i915/dp: Program source OUI on eDP panels

Since we're about to start adding support for Intel's magic HDR
backlight interface over DPCD, we need to ensure we're properly
programming this field so that Intel specific sink services are exposed.
Otherwise, 0x300-0x3ff will just read zeroes.

We also take care not to reprogram the source OUI if it already matches
what we expect. This is just to be careful so that we don't accidentally
take the panel out of any backlight control modes we found it in.

v2:
* Add careful parameter to intel_edp_init_source_oui() to avoid
  re-writing the source OUI if it's already been set during driver
  initialization

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: thaytan@noraisin.net
Cc: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201204223603.249878-2-lyude@redhat.com