drm/bochs: move ttm_bo_(un)reserve calls into bochs_bo_{pin, unpin}

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20190111053752.4004-13-kraxel@redhat.com