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 <ian.d.romanick@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
(cherry picked from commit 46386816a77c6e5751a0685776c0a6320f46f8fe)

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>

The original patch needed some backporting for the old tree.