Revert "drm/i915: Don't use VBT for detecting DPCD backlight controls"

Revert "drm/i915: Don't use VBT for detecting DPCD backlight controls"

This reverts commit 833d7790dc654ced8d132d3194e1c713ac6592dc.

So, turns out that this ended up just breaking things. While many
laptops incorrectly advertise themselves as supporting PWM backlight
controls, they actually will only work with DPCD backlight controls.
Unfortunately, it also seems there are a number of systems which
advertise DPCD backlight controls in their eDP DPCD but don't actually
support them. Talking with some laptop manufacturers has shown it might
be possible to probe this support via the EDID (!?!?) but I haven't been
able to confirm that this would work on any other manufacturer's
systems.

So in the mean time, we'll just revert this commit for now and go back
to the old way of doing things.

Fixes: 833d7790dc65 ("drm/i915: Don't use VBT for detecting DPCD backlight controls")
Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200204192823.111404-2-lyude@redhat.com