accel/amdxdna: Add a function to walk hardware contexts

Walking hardware contexts created by a process is duplicated in multiple
spots. Add a function, amdxdna_hwctx_walk(), and replace all spots.

hwctx_srcu and dev_lock are good enough to protect hardware context list.
Remove hwctx_lock.

Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
Link: https://lore.kernel.org/r/20250815171634.3417487-1-lizhi.hou@amd.com