drm/tilcdc: use devm_of_find_backlight

Look up backlight device using devm_of_find_backlight().
This simplifies the code and prevents us from hardcoding
the node name in the driver.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Jyri Sarha <jsarha@ti.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200514191001.457441-3-sam@ravnborg.org