intel: Only worry about db format if one attached

An undefined depth buffer format is okay if there is no
depth buffer attached. This commit changes the existing assert
for invalid format to allow that if no depth buffer attached.
diff --git a/icd/intel/cmd_pipeline.c b/icd/intel/cmd_pipeline.c
index ec85aa3..57bdc0f 100644
--- a/icd/intel/cmd_pipeline.c
+++ b/icd/intel/cmd_pipeline.c
@@ -400,7 +400,7 @@
             format = GEN6_ZFORMAT_D32_FLOAT;
             break;
         default:
-            assert(!"unknown depth format");
+            assert(!cmd->bind.att.ds); // Must have valid format if ds attached
             format = 0;
             break;
         }