drm/amd/display: Unlock on error path in dm_handle_mst_sideband_msg_ready_event()

This error path needs to unlock the "aconnector->handle_mst_msg_ready"
mutex before returning.

Fixes: 4f6d9e38 ("drm/amd/display: Add polling method to handle MST reply packet")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c

@@ -706,7 +706,7 @@ void dm_handle_mst_sideband_msg_ready_event(
 
 			if (retry == 3) {
 				DRM_ERROR("Failed to ack MST event.\n");
-				return;
+				break;
 			}
 
 			drm_dp_mst_hpd_irq_send_new_request(&aconnector->mst_mgr);