backlight: pwm: Handle EPROBE_DEFER while requesting the PWM

When trying to request the PWM device with devm_pwm_get(), the EPROBE_DEFER
flag is not handled properly. It can lead to the PWM not being found.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>