drm: virtio: use kmem_cache

Just use kmem_cache instead of rolling
our own, limited implementation.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1488377348-5006-1-git-send-email-kraxel@redhat.com