drm/amd/display: add missing function pointer for DCN321 resource

[why]
acquire_free_pipe_as_secondary_opp_head function pointer is not
assigned. This causes ODM mode to be disabled after enabling
windowed MPO ODM capability on DCN321.

[how]
assign the correct function to the function pointer.
Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn321/dcn321_resource.c

@@ -1589,6 +1589,7 @@ static struct resource_funcs dcn321_res_pool_funcs = {
 	.calculate_wm_and_dlg = dcn32_calculate_wm_and_dlg,
 	.populate_dml_pipes = dcn32_populate_dml_pipes_from_context,
 	.acquire_free_pipe_as_secondary_dpp_pipe = dcn32_acquire_free_pipe_as_secondary_dpp_pipe,
+	.acquire_free_pipe_as_secondary_opp_head = dcn32_acquire_free_pipe_as_secondary_opp_head,
 	.release_pipe = dcn20_release_pipe,
 	.add_stream_to_ctx = dcn30_add_stream_to_ctx,
 	.add_dsc_to_stream_resource = dcn20_add_dsc_to_stream_resource,