Fifo: Make g_bSkipCurrentFrame a TU-local variable

This is only ever queried, making it a global isn't necessary.

Source/Core/VideoCommon/OpcodeDecoding.cpp

@@ -277,7 +277,7 @@ u8* Run(DataReader src, u32* cycles, bool in_display_list)
 					(cmd_byte & GX_PRIMITIVE_MASK) >> GX_PRIMITIVE_SHIFT,
 					num_vertices,
 					src,
-					Fifo::g_bSkipCurrentFrame,
+					Fifo::WillSkipCurrentFrame(),
 					is_preprocess);
 
 				if (bytes < 0)