drm/amd/display: Align the returned error code with legacy DP

[Why]
For usb4 connector, AUX transaction is handled by dmub utilizing a differnt
code path comparing to legacy DP connector. If the usb4 DP connector is
disconnected, AUX access will report EBUSY and cause igt@kms_dp_aux_dev
fail.

[How]
Align the error code with the one reported by legacy DP as EIO.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -979,6 +979,11 @@ int dm_helper_dmub_aux_transfer_sync(
 		struct aux_payload *payload,
 		enum aux_return_code_type *operation_result)
 {
+	if (!link->hpd_status) {
+		*operation_result = AUX_RET_ERROR_HPD_DISCON;
+		return -1;
+	}
+
 	return amdgpu_dm_process_dmub_aux_transfer_sync(ctx, link->link_index, payload,
 			operation_result);
 }