samsung: panel: support backlight device phandle

The search order to get backlight control method:
* backlight device specified by "backlight" property in device tree
* set_brightness() callback specified in panel driver
* generic mipi dcs brightness command

Bug: 195449766
Test: brightness can be changed by moving brightness slider
Signed-off-by: Shiyong Li <shiyongli@google.com>
Change-Id: Ieeec3e8bd3b5e120d8f236cbb3d48e76bf9a76c8