drm/omap: panel-tpo-td028ttec1: add backlight support

This panel has a backlight, so fetch it from devicetree using the
corresponding property as documented in panel-common.txt. It is
implemented the same way as in panel-dpi.c
This ensures the backlight is also disabled when the display is
turned off like when doing xset dpms force off.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>