isl: Fill in MOCS for NULL depth, stencil, and HiZ buffers.

isl: Fill in MOCS for NULL depth, stencil, and HiZ buffers.

We'd like to add safeguards against accidental use of MOCS 0 (uncached),
which can have large performance implications.  One case where we use
MOCS of 0 is SURFTYPE_NULL depth, stencil, and HiZ buffers, where MOCS
really shouldn't matter, as there's no actual surface to be cached.

That said, it should be harmless to set MOCS for these null surfaces.
We now set the one provided in info->mocs regardless of whether any
buffers actually exist or not.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13480>