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>