drm/amdkfd: potential NULL dereference in kfd_set/reset_event()

If lookup_event_by_id() returns a NULL "ev" pointer then the
spin_lock(&ev->lock) will crash.  This was detected by Smatch:

    drivers/gpu/drm/amd/amdgpu/../amdkfd/kfd_events.c:644 kfd_set_event()
    error: we previously assumed 'ev' could be null (see line 639)

Fixes: 5273e82c ("drm/amdkfd: Improve concurrency of event handling")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>