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

The X1 Extreme is one of the systems that lies about which backlight
interface that it uses in its VBIOS as PWM backlight controls don't work
at all on this machine. It's possible that this panel could be one of
the infamous ones that can switch between PWM mode and DPCD backlight
control mode, but we haven't gotten any more details on this from Lenovo
just yet. For the time being though, making sure the backlight 'just
works' is a bit more important.

So, add a quirk to force DPCD backlight controls on for these systems
based on EDID (since this panel doesn't appear to fill in the device ID).
Hopefully in the future we'll figure out a better way of probing this.

Changes since v2:
* The bugzilla URL is deprecated, bug reporting happens on gitlab now.
  Update the messages we print to reflect this
* Also, take the opportunity to move FDO_BUG_URL out of i915_utils.c and
  into i915_utils.h so that other places which print things that...