drm/i915: switch off backlight for backlight class 0 brightness

Make backlight class sysfs brightness 0 value switch off the backlight
for connectors that have the backlight_power callback defined. For eDP,
this has the similar caveats regarding power savings as bl_power as only
the power sequencer backlight control is switched off.
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Reviewed_by: Clinton Taylor <Clinton.A.Taylor@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_panel.c

@@ -986,7 +986,8 @@ static int intel_backlight_device_update_status(struct backlight_device *bd)
 	 */
 	if (panel->backlight.enabled) {
 		if (panel->backlight_power) {
-			bool enable = bd->props.power == FB_BLANK_UNBLANK;
+			bool enable = bd->props.power == FB_BLANK_UNBLANK &&
+				bd->props.brightness != 0;
 			panel->backlight_power(connector, enable);
 		}
 	} else {