UBUNTU: SAUCE: drm/i915: Force DPCD backlight mode on X1 Extreme 2nd Gen 4K AMOLED...

UBUNTU: SAUCE: drm/i915: Force DPCD backlight mode on X1 Extreme 2nd Gen 4K AMOLED panel

BugLink: https://bugs.launchpad.net/bugs/1856134
Annoyingly, the VBT on the ThinkPad X1 Extreme 2nd Gen indicates that
the system uses plain PWM based backlight controls, when in reality the
only backlight controls that work are the standard VESA eDP DPCD
backlight controls.

Honestly, this makes me wonder how many other systems have these issues
or lie about this in their VBT. Not sure we have any good way of finding
out until panels like this become more common place in the laptop
market. For now, just add a DRM DP quirk to indicate that this panel is
telling the truth and is being a good LCD.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=112376
Signed-off-by: Lyude Paul <lyude@redhat.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
(backported from https://patchwork.freedesktop.org/patch/342166/?series=69914&rev=4)
Signed-off-by: AceLan Kao <acelan.kao@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>