drm/omapdrm: Use of_find_backlight helper

Replace of_find_backlight_by_node and of the code around it
with of_find_backlight helper to avoid repetition of code.

Reviewed-by: Noralf Trønnes <noralf@tronnes.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/eb914313f9d0e58d81572ccd3c718a573a891bd7.1516810726.git.meghana.madhyastha@gmail.com