drm/amdkfd: fix a resource leakage issue

The function kfd_lookup_process_by_pasid will increase the reference
count of kfd_process object, its caller should call kfd_unref_process to
decrease the reference count. Otherwise resource leakage will happen.
Signed-off-by: Dennis Li <Dennis.Li@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdkfd/kfd_events.c

@@ -1088,4 +1088,6 @@ void kfd_signal_poison_consumed_event(struct kfd_dev *dev, u32 pasid)
 
 	/* user application will handle SIGBUS signal */
 	send_sig(SIGBUS, p->lead_thread, 0);
+
+	kfd_unref_process(p);
 }