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

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

drm_bridge_get_prev_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/20250709-drm-bridge-alloc-getput-drm_bridge_get_prev_bridge-v1-1-34ba6f395aaa@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>