drm/bridge: ti-sn65dsi86: Make use of pwmchip_parent() accessor

struct pwm_chip::dev is about to change. To not have to touch this
driver in the same commit as struct pwm_chip::dev, use the accessor
function provided for exactly this purpose.

Acked-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/10a8d55110fc48a4759e65cc19556858587e94cc.1707900770.git.u.kleine-koenig@pengutronix.de


Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>