drm/bridge: lock the encoder chain in scoped for_each loops

drm/bridge: lock the encoder chain in scoped for_each loops

drm_for_each_bridge_in_chain_scoped() and
drm_for_each_bridge_in_chain_from() currently get/put the bridge at each
iteration. But they don't protect the encoder chain, so it could change
(bridges added/removed) while some code is iterating over the list
itself. Such code can then derail on incorrect pointers.

To make iterations safe, augment these for_each macros to lock the encoder
chain mutex at the beginning and unlock it at the end of the loop (be it at
the end of the list, or earlier due to a 'break' or 'return' statement).

This change requires more operations when starting and ending the loop. To
avoid making the macros even more complex, move these operations to helper
functions. Also remname some of the existing helper functions for
consistency.

Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Link: https://patch.msgid.link/20260324-drm-bridge-alloc-encoder-chain-mutex-v5-4-8bf786c5c7e6@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>