drm/amd/display: add DP protocol version

[Why]
We want to know DP protocol version

[How]
In DC create we initialize a cap to indicate the max
DP protocol version supported

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>