drm/virtio: Don't attach GEM to a non-created context in gem_object_open()

The vfpriv->ctx_id is always initialized to a non-zero value. Check whether
context was created before attaching GEM to this context ID. This left
unnoticed previously because host silently skips attachment if context
doesn't exist, still we shouldn't do that for consistency.

Fixes: 086b9f27 ("drm/virtio: Don't create a context with default param if context_init is supported")
Cc: <stable@vger.kernel.org> # v6.14+
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Link: https://lore.kernel.org/r/20250401123842.2232205-1-dmitry.osipenko@collabora.com