arm64: tegra: Remove unused backlight-boot-off property

The backlight-boot-off property was proposed as a patch, but ended not
being accepted since different solution was already in the place:

    https://patchwork.kernel.org/project/linux-arm-kernel/patch/1406806970-12561-1-git-send-email-thierry.reding@gmail.com/#21327479



Signed-off-by: David Heidelberg <david@ixit.cz>
Signed-off-by: Thierry Reding <treding@nvidia.com>