drm/debugfs: add top-level 'bridges' file showing all added bridges

drm/debugfs: add top-level 'bridges' file showing all added bridges

The global bridges_list holding all the bridges between drm_bridge_add()
and drm_bridge_remove() cannot be inspected via debugfs. Add a file showing
it.

To avoid code duplication, move the code printing a bridge info to a common
function.

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250226-drm-debugfs-show-all-bridges-v8-2-bb511cc49d83@bootlin.com
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>