drm/omap: panel-dsi-cm: add external backlight support

Droid 4 has a command mode DSI panel, which does not have/use
DSI based backlight support. This adds proper support for this
using a backlight phandle property, which follows the common
panel binding.

If no backlight phandle is found, it is assumed, that the
native backlight should be used instead. This is used by
the Nokia N950.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>