dri: fromPlanar() can return NULL as a valid result

dri: fromPlanar() can return NULL as a valid result

It was assumed that fromPlanar() could return NULL to mean
that the planar image is the same as the parent DRI image.
That assumption wasn't made everywhere though.

Let's fix things and make sure that all callers understand
a NULL result

Signed-off-by: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Daniel Stone <daniels@collabora.com>