drm/bridge: panel: Introduce drmm_panel_bridge_add

Unlike what can be found for other entities, there's no DRM-managed
function to create a panel_bridge instance from a panel.

Let's introduce one.

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