drm/amd/display: Do not retain link settings

Do not retrain link settings if lane count and link rate are both
unknown.  Causes driver to be stuck reading VBIOS register after
removing emulated connection.

Signed-off-by: Samson Tam <Samson.Tam@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>