drm/xe/client: remove bogus rcu list usage

We use plain spinlock to protect readers and writers, so there is no
actual RCU here. Rather use the more appropriate non-rcu list based API.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240318093431.21075-3-matthew.auld@intel.com