drm/amd/display: remove stream_func vtable

Signed-off-by: Tony Cheng <tony.cheng@amd.com>
Reviewed-by: Sun peng Li <Sunpeng.Li@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>