drm/bridge: drm_bridge_attach: lock the encoder chain mutex during insertion

drm/bridge: drm_bridge_attach: lock the encoder chain mutex during insertion

drm_bridge_attach() modifies the encoder bridge chain, so take a mutex
around such operations to allow users of the chain to protect themselves
from chain modifications while iterating.

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-v5-3-8bf786c5c7e6@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>