drm/tilcdc: panel: Fix backlight devicetree support

The current backlight support is broken; the driver expects a backlight-class
in the panel devicetree node. Fix this by implementing it properly, getting
an optional backlight from a phandle.

This shouldn't cause any backward-compatibility DT issue because the current
implementation doesn't work and is not even documented.

Tested-by: Darren Etheridge <detheridge@ti.com>
Tested-by: Johannes Pointner <johannes.pointner@br-automation.com>
Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Dave Airlie <airlied@redhat.com>

Documentation/devicetree/bindings/drm/tilcdc/panel.txt

@@ -18,6 +18,9 @@ Required properties:
    Documentation/devicetree/bindings/video/display-timing.txt for display
    timing binding details.
 
+Optional properties:
+- backlight: phandle of the backlight device attached to the panel
+
 Recommended properties:
  - pinctrl-names, pinctrl-0: the pincontrol settings to configure
    muxing properly for pins that connect to TFP410 device
@@ -29,6 +32,8 @@ Example:
 		compatible = "ti,tilcdc,panel";
 		pinctrl-names = "default";
 		pinctrl-0 = <&bone_lcd3_cape_lcd_pins>;
+		backlight = <&backlight>;
+
 		panel-info {
 			ac-bias           = <255>;
 			ac-bias-intrpt    = <0>;