Fix stencil setup in Metal

We could have a case where stencil is disabled but the framebuffer
has a stencil buffer, and the framebuffer and pipeline must match.

Bug: skia:8243
Change-Id: I460543392ffaf7198d693fd067fbdc96cda71fd3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/197764
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>