Fixed obscure level-2 packing / general stride bug.

Details:
- Fixed a bug in certain structured level-2 operations that manifested
  only when the structured matrix was provided to BLIS as matrix stored
  with general stride. The bug was introduced in c472993b when the
  densify field was removed from the packm control tree node and
  associated APIs. Since then, the packed object was unconditionally
  marked with an uplo field of BLIS_DENSE. This is fine for level-3
  operations where micro-panels are always densified, but in level-2
  contexts, the underlying unblocked variant (fused or unfused) of
  structured operations (e.g. trmv) still needs to know whether to
  execute its "lower" or "upper" branches of code. Since this field
  was unconditionally being set to BLIS_DENSE, the unblocked variants
  were always executed the "else" branch, which happened to be the
  "lower" case code. Thus, running an upper case produced the wrong
  answer. This most obviously manifested in the form of failures for
  trmm, trmm3, and trsm in the test suite.
  The bug was fixed by setting the packed object's uplo field to
  BLIS_DENSE only if the schema indicated that micro-panels were to be
  packed. Otherwise, we can assume we are packing to regular row or
  column storage, as is the case with level-2 packing. Thanks to
  Francisco Igual for reporting the testsuite failures and ultimately
  leading us to this bug.