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>