[Why]
We skip dsc_validation on pipes that are underlays, but in the
acquire_dsc code we don't have this check.

In certain conditions (when underlay pipe index is lower) we will assign
the dsc resource to the underlay pipe and skip the base pipe.

Now during dsc_validation we will skip the underlay pipe (this has the
dsc resource) but try to validate the base pipe(this doesn't have a dsc
resource) due to this mismatch we hit a NULLPTR

[How]
In the acquire_dsc add a check for underlay pipe so we
don't acquire a dsc resource for this pipe. This will match the
acquire/validation conditions.
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Hersen Wu <Hersenxs.Wu@amd.com>
Acked-by: Praful Swarnakar <Praful.Swarnakar@amd.com>
Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>