drm/amdkfd: fix kfd_suspend_all_processes

Flush delayed restore work in kfd_suspend_all_queues instead of
cancelling. Cancelling the work before it runs results in the queues
becoming permanently disabled. Flushing the work ensures that the
queue suspend/resume state stays balanced.

Signed-off-by: Jonathan Kim <jonathan.kim@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>