drm/amd/display: initialize link_srv in virtual env

[why]
when refactoring to link_srv, we did not include the
virtual environment case where dc_construct_ctx gets
called instead of dc_construct

[how]
add initialize link_srv to dc_construct_ctx as well

Reviewed-by: Leo Ma <hanghong.ma@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Martin Leung <Martin.Leung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>