iris: don't create staging resources larger than half the aperture

iris: don't create staging resources larger than half the aperture

This is a port of crocus's f1c1fcfd (crocus: don't create staging
resources > half aperture).

I chose to use the whole aperture here rather than 3/4 of it, mostly
to avoid adding another legacy field for aperture stuff.  It's close
enough to work.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/2104
Reviewed-by: Dave Airlie <airlied@redhat.com>
Tested-by: Mark Janes markjanes@swizzler.org
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18311>