drm/amd/display: Add Missing HPO Stream Encoder Function Hook

[Why]
configure_dp_hpo_throttled_vcp_size() was missing promotion before, but it was covered by
not calling the missing function hook in the old interface hpo_dp_link_encoder->funcs.

Recent refactor replaces with new caller link_hwss->set_throttled_vcp_size
which needs that hook, and that causes null ptr hang.

Signed-off-by: Fangzhi Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>