drm/encoder: add mutex to protect the bridge chain

drm/encoder: add mutex to protect the bridge chain

The per-encoder bridge chain is currently assumed to be static once it is
fully initialized. Work is in progress to add hot-pluggable bridges,
breaking that assumption.

With bridge removal, the encoder chain can change without notice, removing
tail bridges. This can be problematic while iterating over the chain.

Add a mutex to be taken whenever looping or changing the encoder chain.

Reviewed-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Link: https://patch.msgid.link/20260324-drm-bridge-alloc-encoder-chain-mutex-v5-1-8bf786c5c7e6@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>