drm/bridge: prevent encoder chain changes while iterating with list_for_each_entry_re...

drm/bridge: prevent encoder chain changes while iterating with list_for_each_entry_reverse()

These loops in drm_bridge.c iterate over the encoder chain using
list_for_each_entry_reverse(), which does not prevent changes to the bridge
chain while iterating over it.

Take the encoder chain mutex while iterating to avoid chain changes while
iterating.

All the "simple" loops are converted. drm_atomic_bridge_chain_pre_enable()
and drm_atomic_bridge_chain_post_disable() are handled by a separate
commit.

Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Link: https://patch.msgid.link/20260324-drm-bridge-alloc-encoder-chain-mutex-v5-6-8bf786c5c7e6@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>