drm/amdkfd: fix race condition in kfd_wait_on_events

Add the waiters to the wait queue during initialization, while holding the
event spinlock. Otherwise the waiter will not get activated if the event
signals before being added to the wait queue.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Philip <Yang&lt;Philip.Yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>