pwm: mtk-disp: Simplify using devm_pwmchip_add()

With devm_pwmchip_add() pwmchip_remove() can be dropped from the remove
callback. Then the remove callback is empty and can go away, too. With
mtk_disp_pwm_remove() the last user of platform_get_drvdata() is gone and
so platform_set_drvdata() can be dropped, too.

Also use dev_err_probe() for simplified (and improved) error reporting.

Link: https://lore.kernel.org/r/20230929161918.2410424-6-u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>