drm/bridge: move bridges_show logic from drm_debugfs.c

drm/bridge: move bridges_show logic from drm_debugfs.c

In preparation to expose more info about bridges in debugfs, which will
require more insight into drm_bridge data structures, move the bridges_show
code to drm_bridge.c.

Suggested-by: Jani Nikula <jani.nikula@linux.intel.com>
Suggested-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
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-v8-1-bb511cc49d83@bootlin.com
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>