drm/virtio: make sure context is created in gem open

drm/virtio: make sure context is created in gem open

The context might still be missing when DRM_IOCTL_PRIME_FD_TO_HANDLE is
the first ioctl on the drm_file.

Fixes: ec15ca83305a ("drm/virtio: enqueue virtio_gpu_create_context after the first 3D ioctl")
Cc: Gurchetan Singh <gurchetansingh@chromium.org>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20210107210726.269584-1-olvaffe@gmail.com
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>