drm/bridge: get the bridge returned by drm_bridge_chain_get_first_bridge()

drm/bridge: get the bridge returned by drm_bridge_chain_get_first_bridge()

drm_bridge_chain_get_first_bridge() returns a bridge pointer that the
caller could hold for a long time. Increment the refcount of the returned
bridge and document it must be put by the caller.

Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20250708-drm-bridge-alloc-getput-drm_bridge_chain_get_first_bridge-v9-2-db1ba3df7f58@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>