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