gl-renderer: Do not attach the first buffer twice

When gl_renderer_attach is called, surface->buffer_ref.buffer points to the buffer.
So if the surface state needs to be created, we have :
gl_renderer_attach -> get_surface_state -> gl_renderer_create_surface -> gl_renderer_attach

Fixes: 895b1fdc ("gl-renderer: Attach buffer during surface state creation if possible")
Signed-off-by: Loïc Yhuel <loic.yhuel@softathome.com>