drm: move pinning/unpinning to buffer attach

This allows importing bo's to own device to work without requiring that the buffer is pinned in GART.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>