Skip loading unknown XF registers in the FIFO player

This avoids some warnings, which were originally fixed by ignoring loads with a value of zero (see 636bedb207 / #3242).

Note that FifoCI will report some changes, but only on the first frame; these seem to be timing related as they don't happen if a different write is used to replace skipped ones.

Source/Core/VideoCommon/XFStructs.cpp

@@ -195,8 +195,7 @@ static void XFRegWritten(int transferSize, u32 baseAddress, DataReader src)
 
     default:
       DolphinAnalytics::Instance().ReportGameQuirk(GameQuirk::USES_UNKNOWN_XF_COMMAND);
-      if (newValue != 0)  // Ignore writes of zero.
-        WARN_LOG_FMT(VIDEO, "Unknown XF Reg: {:x}={:x}", address, newValue);
+      WARN_LOG_FMT(VIDEO, "Unknown XF Reg: {:x}={:x}", address, newValue);
       break;
     }