wined3d: Explicitly pass the depth/stencil surface to context_validate_onscreen_forma...

wined3d: Explicitly pass the depth/stencil surface to context_validate_onscreen_formats().

This avoids switching to offscreen rendering in context_apply_blit_state() for
a depth buffer that isn't going to be used anyway. This switch would be
problematic for blits to the frontbuffer, since it's always onscreen. I.e.,
we'd be blitting to the screen, but with offscreen transforms.