st/mesa: don't choose DXT formats if we can't do DXT compression

If we call gl[Copy]TexImage2D() with a generic compression format
(e.g. intFormat=GL_COMPRESSED_RGBA) we can't choose a DXT format if
we don't have the external DXT compression library.

We weren't actually enforcing this before since the
pipe_screen::is_format_supported(DXT) query has no dependency on
the DXT compression library.

Now if we're given a generic compressed format and we can't do DXT
compression we'll fall back to a non-compressed format.

v2: use util_format_is_s3tc() function and add more comments about
the allow_dxt parameter.

Note: This is a candidate for the stable branches.

Reviewed-by: Jose Fonseca <[email protected]>

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