backlight: pwm_bl: Avoid backlight flicker when probed from DT

If the driver is probed from the device tree, and there is a phandle
property set on it, and the enable GPIO is already configured as output,
and the backlight is currently disabled, keep it disabled.
If all these conditions are met, assume there will be some other driver
that can enable the backlight at the appropriate time.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>