drm/nouveau: Add support to control backlight using bl_power for nva3.

Summary:

* Add support to turn on/off backlight when changing values in bl_power
  file. This is achieved by using function backlight_get_brightness()
  in nva3_set_intensity to get current brightness.

Test plan:

* Turn off:
echo 1 > /sys/class/backlight/nv_backlight/bl_power

* Turn on:
echo 0 > /sys/class/backlight/nv_backlight/bl_power

Signed-off-by: Antonio Gomes <antoniospg100@gmail.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221104220424.41164-1-antoniospg100@gmail.com