mesa: only test cube face widths in _mesa_test_texobj_completeness()
mesa: only test cube face widths in _mesa_test_texobj_completeness()

As Eric pointed out, we know the cube faces are square at this point
so we only need to test the texture widths for consistency.

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

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