drm/tests: bridge: convert to devm_drm_bridge_alloc() API

drm/tests: bridge: convert to devm_drm_bridge_alloc() API

Use the new DRM bridge allocation API, which is the only supported now, for
the kunit tests.

This change is more massive than for the typical DRM bridge driver because
struct drm_bridge_init_priv currently embeds a struct drm_bridge, which is
not supported anymore. We now have to use devm_drm_bridge_alloc() to
dynamically allocate a "private driver struct", which is a bit awkward here
because there is no real bridge driver. Thus let's add a "dummy" DRM bridge
struct to represent it.

As a nice cleanup we can now move the enable_count and disable_count
members, which are counting bridge-specific events, into the new "private
driver struct" (and avoid adding new unnecessary indirections).

Also add a trivial bridge_to_dummy_bridge() function just like many drivers
do.

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Link: https://lore.kernel.org/r/20250606-drm-bridge-alloc-doc-test-v9-1-b5bf7b43ed92@bootlin.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>