drm/panthor: Always wait after sending a command to an AS

drm/panthor: Always wait after sending a command to an AS

There's currently no situation where we want to issue a command to an
AS and not wait for this command to complete. The wait is either
explicitly done (LOCK, UNLOCK) or it's missing (UPDATE). So let's
turn write_cmd() into as_send_cmd_and_wait() that has the wait after
a command is sent.

v2:
- New patch

v3:
- Collect R-b

v4:
- No changes

Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patch.msgid.link/20251128084841.3804658-2-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>