gpu-compute: init valid field of GpuTlbEntry in default ctor

valid field for GpuTlbEntry is not set in the default ctor, which can
lead to strange behavior, and is also flagged by UBSAN.

src/gpu-compute/gpu_tlb.hh

@@ -68,7 +68,7 @@ namespace X86ISA
         GpuTlbEntry(Addr asn, Addr _vaddr, Addr _paddr, bool _valid)
           : TlbEntry(asn, _vaddr, _paddr, false, false), valid(_valid) { }
 
-        GpuTlbEntry() : TlbEntry() { }
+        GpuTlbEntry() : TlbEntry(), valid(false) { }
 
         bool valid;
     };