drm/vgem: Enable dmabuf import interfaces

Enable the GEM dma-buf import interfaces in addition to the export
interfaces. This lets vgem be used as a test source for other allocators
(e.g. Ion).

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1493923548-20878-4-git-send-email-labbott@redhat.com