drm/bridge: ti-sn65dsi86: Make use of devm_pwmchip_alloc() function

This prepares the pwm driver of the ti-sn65dsi86 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.

Acked-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/a56cbaf049f5f23c0e0fe36b0799dd20189675e0.1707900770.git.u.kleine-koenig@pengutronix.de


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