drm/bridge: panel: Introduce drmm_of_get_bridge

Unlike what can be found for other DRM entities, we don't have a
DRM-managed function equivalent to devm_drm_of_get_bridge().

Let's create it.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>