drm/amd/display: add query HPD interface.

[Why]
current dc_link_detect function is not only detection but also update some link data.

[How]
added a pure get HPD state function.

Signed-off-by: Chiawen Huang <chiawen.huang@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>