drm/amd/display: Fix construct_phy with MXM connector

[Why/How]
The call to construct_phy will fail in cases where connector type is
MXM, and the dc_link won't be properly created/initialized.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Ilya Bakoulin <Ilya.Bakoulin@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>