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

drivers/gpu/drm/nouveau/nouveau_backlight.c

@@ -264,7 +264,11 @@ nva3_set_intensity(struct backlight_device *bd)
 	u32 div, val;
 
 	div = nvif_rd32(device, NV50_PDISP_SOR_PWM_DIV(or));
-	val = (bd->props.brightness * div) / 100;
+
+	val = backlight_get_brightness(bd);
+	if (val)
+		val = (val * div) / 100;
+
 	if (div) {
 		nvif_wr32(device, NV50_PDISP_SOR_PWM_CTL(or),
 			  val |