drm/amd/display: Add function pointer for validate bw usb4

[WHY]
In order to follow the new protocol of calling link functions

[HOW]
Add the function pointer to the link_srv

Reviewed-by: Kshitij Bhardwaj <kshitij.bhardwaj1@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Mustapha Ghaddar <mghaddar@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>