blorp: Fill in MOCS even for SURFTYPE_NULL surfaces.

blorp: Fill in MOCS even for SURFTYPE_NULL surfaces.

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 surfaces, 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 can just assume a generic MOCS for internal buffers.

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