drm/bridge: parade-ps8640: switch to devm_drm_of_get_bridge

The function "drm_of_find_panel_or_bridge" has been deprecated in
favor of "devm_drm_of_get_bridge".

Switch to the new function and reduce boilerplate.

Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220228183131.23494-1-jose.exposito89@gmail.com