st/mesa: restore original last_layer comparison

Commit e648d4a1d1c0c5f70916e38366b863f0bec79a62 changed the original
less-than test to a not-equal test.  This was an effort to save some
memory by switching the texture layout to a non-mipmapped layout when
we mis-guessed about the original layout (thus saving some memory).

However, this causes us to hit a new (apparently broken) code path
when copying the old texture's data to the new texture.  Simply
undo this change for the time being until the other/new bug is fixed.

Fixes fd.o bug 27933.

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