drm/omapdrm: use drm_bridge_chain_get_last_bridge()

drm/omapdrm: use drm_bridge_chain_get_last_bridge()

Use drm_bridge_chain_get_last_bridge() instead of open coding a loop with
two invocations of drm_bridge_get_next_bridge() per iteration.

Besides being cleaner and more efficient, this change is necessary in
preparation for drm_bridge_get_next_bridge() to get a reference to the
returned bridge.

Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20250801-drm-bridge-alloc-getput-drm_bridge_get_next_bridge-v2-4-888912b0be13@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>