drm/tinydrm: Call devres version of of_find_backlight

Call devm_of_find_backlight (the devres version) instead of
of_find_backlight.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Meghana Madhyastha <meghana.madhyastha@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/244cd4d567cb3944a8cb7633715ffc8ac4e1ce83.1516810725.git.meghana.madhyastha@gmail.com