[mlir][sparse] avoid reserving dense storage for ptr/idx

This avoids a rather big bug where we were reserving
dense space for the ptx/idx in the first sparse dimension.
For example, using CSR for a 140874 x 140874 matrix with
3977139 nonzero would reserve the full 19845483876 space.
This revision fixes this for now, but we need to revisit
the reservation heuristic to make this better.

Reviewed By: bixia

Differential Revision: https://reviews.llvm.org/D123166