drm/amd/display: support dynamic HPO DP link encoder allocation

[why]
When there are more DP2.0 RXs connected than the number HPO DP link
encoders we have, we need to dynamically allocate HPO DP link encoder to
the port that needs it.

[how]
Only allocate HPO DP link encoder when it is needed.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>