[graphite] Avoid redundant fields in DepthStencilSettings

Whether or not the tests are enabled is inferred from the write state
and the comparison state. Cleans up the backend depth/stencil state
objects as well.

Bug: 500416105
Change-Id: I1252a26c848a0e3fe536688fec04b374774d89b6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1373996
Reviewed-by: Nathan Sanchez <nathanasanchez@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>