drm/omapdrm: Use backlight_enable/disable helpers

Use backlight_enable/disable helpers instead of changing
the property and calling backlight_update_status for cleaner
and simpler code and also to avoid repetitions.

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/30d5b26c3c7d75de3d2ab3cff9dee67084fc3caf.1516810725.git.meghana.madhyastha@gmail.com