fbdev: nvidia: Use backlight helper

Instead of retrieving the backlight brightness in struct
backlight_properties manually, and then checking whether the backlight
should be on at all, use backlight_get_brightness() which does all
this and insulates this from future changes.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Cc: Antonino Daplas <adaplas@gmail.com>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-fbdev@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Helge Deller <deller@gmx.de>