drm/nouveau/kms/nv50-: Add basic DPCD backlight support for nouveau

This adds support for controlling panel backlights over eDP using VESA's
standard backlight control interface. Luckily, Nvidia was cool enough to
never come up with their own proprietary backlight control interface (at
least, not any that I or the laptop manufacturers I've talked to are aware
of), so this should work for any laptop panels which support the VESA
backlight control interface.

Note that we don't yet provide the panel backlight frequency to the DRM DP
backlight helpers. This should be fine for the time being, since it's not
required to get basic backlight controls working.

For reference: there's some mentions of PWM backlight values in
nouveau_reg.h, but I'm not sure these are the values we would want to use.
If we figure out how to get this information in the future, we'll have the
benefit of more granular backlight control.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Dave Airlie <airlied@gmail.com>
Cc: greg.depoire@gmail.com
Link: https://patchwork.freedesktop.org/patch/msgid/20210514181504.565252-10-lyude@redhat.com