drm/amd/display: Linux Set/Read link rate and lane count through debugfs

expose dc function to be called by linux dm

Signed-off-by: Hersen Wu <hersenxs.wu@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>