FifoPlayer: reduce XF_REGS_SIZE to what we know

This avoids out-of-bounds warnings when replaying FIFO captures.

The value of XF_REGS_SIZE is written into the DFF header and we only
read the min of XF_REGS_SIZE and the header value, so this change is
backward compatible and doesn't break forward compatibility for old
Dolphin versions either.

Source/Core/VideoCommon/XFMemory.h

@@ -288,6 +288,7 @@ struct XFMemory
   u32 unk9[8];                              // 0x1048 - 0x104f
   PostMtxInfo postMtxInfo[8];               // 0x1050 - 0x1057
 };
+static_assert(sizeof(XFMemory) == sizeof(u32) * 0x1058);
 
 extern XFMemory xfmem;