drm/amd/display: determine USB C DP2 mode only when USB DP Alt is enabled

[why]
When display is connected with a native DP port, DP2 mode register value
is a don't care. Driver mistakenly reduce max supported lane count to 2
lane based on the don't care value.

[how]
Add additional check only if USB C DP alt mode is enabled, we will
determine max lane count supported based on current mode.

Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>