i965,iris: Delete misleading HiZ sampling comments

i965,iris: Delete misleading HiZ sampling comments

One comment seems to suggest that MCS (which is needed for compressed
multisampling) can be used to sample from a multisampled depth buffer.
This is not the case. Multisampled depth buffers are sampled without an
auxiliary surface.

Another comment seems to suggest that some depth buffers don't have
corresponding levels in their HiZ buffers. Each main slice *should* have
a corresponding aux slice, but not all of these slices have equal
support for HiZ ops (e.g. ambiguates aren't really supported on
non-8x4-aligned slices).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8853>