drm/virtio: fix memory leak of vfpriv on error return path

The allocation for vfpriv is being leaked on an error return path,
fix this by kfree'ing it before returning.

Detected by CoverityScan, CID#1475380 ("Resource Leak")

Fixes: 6a37c49a

 ("drm/virtio: Handle context ID allocation errors")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20181107203122.6861-1-colin.king@canonical.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>