drm/amd/display: Cleanup MST non-atomic code workaround

[why]
It is not correct to touch aconnector within atomic_check.

[How]
It was added as workaround before, and no longer needed.

Signed-off-by: Jerry (Fangzhi) Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>