drm/bridge: add drm_bridge_unplug() and drm_bridge_enter/exit()

drm/bridge: add drm_bridge_unplug() and drm_bridge_enter/exit()

To allow DRM bridges to be removable, add synchronization functions
allowing to tell when a bridge hardware has been physically unplugged and
to mark a critical section that should not be entered after that.

This is inspired by the drm_dev_unplugged/enter/exit() functions for struct
drm_device.

Suggested-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/all/20250106-vigorous-talented-viper-fa49d9@houat/
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://patch.msgid.link/20251112-drm-bridge-atomic-vs-remove-v3-1-85db717ce094@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>