drm/amd/display: Fix fallback issues for DP LL 1.4a tests

[WHY]
Unlike DP or USBC, the USB4 link does not get its own encoder and
has to share therefore verify_caps is skipped.

[HOW]
Fix the fallback logic for automated tests and take that
into consideration for LT and LS.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Mustapha Ghaddar <mustapha.ghaddar@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>