present/wnmd: Keep pixmap pointer in present_wnmd_clear_window_flip

The comment was incorrect: Any reference held by the window (see
present_wnmd_execute) is in addition to the one in struct present_vblank
(see present_vblank_create). So if we don't drop the latter, the pixmap
will be leaked.

Reviewed-by: Dave Airlie <airlied@redhat.com>