[d3d11] Fix counter value offset in DrawAuto

According to the newly released D3D11.3 functional specification,
we're supposed to subtract the offset of the slot 0 vertex buffer
binding from the counter value.

src/d3d11/d3d11_context.cpp

@@ -1361,7 +1361,7 @@ namespace dxvk {
     EmitCs([=] (DxvkContext* ctx) {
       ctx->drawIndirectXfb(ctrBuf,
         vtxBuf.buffer()->getXfbVertexStride(),
-        0); // FIXME offset?
+        vtxBuf.offset());
     });
   }