drm/virtio: Replace ttm_bo_reference with ttm_bo_get

The function ttm_bo_get acquires a reference on a TTM buffer object. The
function's name is more aligned to the Linux kernel convention of naming
ref-counting function _get and _put.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20180731062127.10131-2-tzimmermann@suse.de


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>