UPSTREAM: backlight: pwm_bl: Fix broken PWM backlight for non-dt platforms

UPSTREAM: backlight: pwm_bl: Fix broken PWM backlight for non-dt platforms

Commit ee65ad0e2a9e ("backlight: pwm_bl: Avoid backlight flicker when
probed from DT") tries to dereference the device of_node pointer
unconditionally, causing a NULL pointer dereference on non-dt platforms.
Fix it by replacing the phandle variable with a node variable and
by checking that for NULL before dereferencing it.

Reported-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Tested-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
(cherry picked from commit 8777078015bb77f0561303b6dea23d40bd9f3053)

Change-Id: If98d57968b7ae64164ae4d5995c8bf3007a35c0c
Signed-off-by: David Wu <david.wu@rock-chips.com>