gl-renderer: Don't re-import dmabufs

My reading of the GL spec is that a dmabuf becomes a sibling to the
EGLImage created from it, and that all updates to the dmabuf will be
propagated to the EGLImage.

A rebind is still required every time the dmabuf content changes,
but this should be satisfied by gl_renderer_attach(), which does
a rebind when the buffer is commit.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>