st/mesa: return appropriate mesa format for ETC texture formats

st/mesa: return appropriate mesa format for ETC texture formats

Even when the backend driver does not support ETC formats, we handle the
decoding into an uncompressed backing texture. However as far as core
mesa is concerned, it's an ETC texture and we should return the relevant
ETC mesa format. This condition can get hit when using glTexStorage to
create the texture object.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Cc: "11.2 12.0" <mesa-stable@lists.freedesktop.org>