drm/nouveau/tmr: fully separate alarm execution/pending lists

Reusing the list_head for both is a bad idea.  Callback execution is done
with the lock dropped so that alarms can be rescheduled from the callback,
which means that with some unfortunate timing, lists can get corrupted.

The execution list should not require its own locking, the single function
that uses it can only be called from a single context.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: stable@vger.kernel.org