drm/bridge: add drm_bridge_is_last()

drm/bridge: add drm_bridge_is_last()

Some code needing to know whether a bridge is the last in a chain currently
call drm_bridge_get_next_bridge(). However drm_bridge_get_next_bridge()
will soon increment the refcount of the returned bridge, which would make
such code more annoying to write.

In preparation for drm_bridge_get_next_bridge() to increment the refcount,
as well as to simplify such code, introduce a simple bool function to tell
whether a bridge is the last in the chain.

Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20250801-drm-bridge-alloc-getput-drm_bridge_get_next_bridge-v2-5-888912b0be13@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>