i965: Fix validation of ETC miptrees.
i965: Fix validation of ETC miptrees.

When comparing to the teximage's format, we have to look at the
format-the-mt-was-created-for not the format-actually-stored-in-the-mt.

Improves glbenchmark 2.1 offscreen test performance 159% +/- 17% (n=3).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=54582
Reviewed-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>
Reviewed-by: Jordan Justen <a79006884ba9cf5f4f7fcbb92a15b208780d8c97@intel.com>

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