drm/virtio: Fix missed dmabuf unpinning in error path of prepare_fb()

Correct error handling in prepare_fb() to fix leaking resources when
error happens.

Fixes: 4a696a2e ("drm/virtio: Add prepare and cleanup routines for imported dmabuf obj")
Cc: <stable@vger.kernel.org> # v6.14+
Acked-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://lore.kernel.org/r/20250401123842.2232205-2-dmitry.osipenko@collabora.com