pwm: mtk-disp: Make use of devm_pwmchip_alloc() function

pwm: mtk-disp: Make use of devm_pwmchip_alloc() function

This prepares the pwm-mtk-disp driver to further changes of the pwm core
outlined in the commit introducing devm_pwmchip_alloc(). There is no
intended semantical change and the driver should behave as before.

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/a70bf253dc41c61d9b901423710c97f235e372c3.1707900770.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>