drm/amd/display: Remove drm_modeset_lock in MST code

Locks are no longer needed since the drm framework takes care of
correct locking.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Jerry Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>