In order to avoid invalidating system memory for "converted" surfaces.

Fixes a regression introduced by commit 56128d18 ("wined3d: Drop the
special case for "converted" surfaces in wined3d_surface_blt().").
Signed-off-by: Józef Kucia <jkucia@codeweavers.com>
Signed-off-by: Henri Verbeet <hverbeet@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>