displayport: fall through to DSC when MST generic check fails at intermediate hops

The PBN pre-check only validates local (source) link bandwidth before
skipping DSC. In MST daisy-chain or hub topologies, an intermediate
branch link may have fewer timeslots than the local link. When
compoundQueryAttachMSTGeneric fails at an intermediate branch, the mode
is rejected without trying DSC — but DSC compression would reduce the
PBN enough to fit through the bottleneck.

Instead of returning the generic result directly, check if it succeeded:
if so, return true (DSC unnecessary). If not, fall through to the DSC
path which tries 10 bpp then 8 bpp max compression as fallback.

Signed-off-by: Cole Leavitt <cole@unwrap.rs>

Author: coleleavitt

src/common/displayport/src/dp_connectorimpl.cpp

@@ -1417,20 +1417,27 @@ bool ConnectorImpl::compoundQueryAttachMST(Group * target,
             unsigned base_pbn, slots, slots_pbn;
             localInfo.lc.pbnRequired(localInfo.localModesetInfo,
                                      base_pbn, slots, slots_pbn);
-
             if (compoundQueryLocalLinkPBN + slots_pbn <=
                 localInfo.lc.pbnTotal())
             {
                 //
                 // The uncompressed mode fits within available local link PBN.
-                // Skip the DSC path and proceed directly to the full generic
-                // validation (watermark, per-device bandwidth). If the generic
-                // check fails for non-bandwidth reasons, the mode is not
-                // supportable regardless of DSC.
+                // Try the full generic validation (watermark, per-device
+                // bandwidth at intermediate MST branches). If it succeeds,
+                // DSC is unnecessary.
+                //
+                if (compoundQueryAttachMSTGeneric(target, modesetParams,
+                                                  &localInfo, pDscParams,
+                                                  pErrorCode))
+                {
+                    return true;
+                }
+                //
+                // Generic validation failed — possibly due to insufficient
+                // bandwidth at an intermediate MST branch link. DSC can
+                // reduce PBN enough to fit through the bottleneck, so fall
+                // through to the DSC path below.
                 //
-                return compoundQueryAttachMSTGeneric(target, modesetParams,
-                                                     &localInfo, pDscParams,
-                                                     pErrorCode);
             }
         }