virglrenderer: promote apis required by venus to stable
Let me send this out separately before Venus release.
- bump stable VIRGL_RENDERER_CALLBACKS_VERSION to 3
- write_context_fence
- get_server_fd
- promote a couple renderer flags
- VIRGL_RENDERER_VENUS
- VIRGL_RENDERER_NO_VIRGL
- VIRGL_RENDERER_ASYNC_FENCE_CB
- VIRGL_RENDERER_RENDER_SERVER
- promote 2 renderer apis
- virgl_renderer_resource_import_blob
- virgl_renderer_context_create_fence