backlight: pwm_bl: Drop support for legacy PWM probing

There is no in-tree user left which relies on legacy probing. So drop
support for it which removes another user of the deprecated
pwm_request() function.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20221117072151.3789691-1-u.kleine-koenig@pengutronix.de