r600: be more exact in vb size calculation

to make kernel cs checker happier, last attribs don't need full stride of
space. Calculate as count-1*stride + size of attrib

src/mesa/drivers/dri/r600/r700_chip.c

@@ -176,7 +176,8 @@ static void r700SetupVTXConstants(GLcontext  * ctx,
     }
     else
     {
-        nVBsize = paos->count * pStreamDesc->stride;
+        nVBsize = (paos->count - 1) * pStreamDesc->stride
+                  + pStreamDesc->size * getTypeSize(pStreamDesc->type);
     }
 
     uSQ_VTX_CONSTANT_WORD0_0 = paos->offset;