i965: Assert that the scratch spaces are in range.

i965: Assert that the scratch spaces are in range.

I don't know that anything actually guarantees this, but if we exceed
the limits, we may end up overflowing and trashing random buffers that
happen to be nearby in the VMA space, leading to rendering corruption,
hangs, or worse.

We should really fix this properly.  However, the pitfall has existed
for ages, so for now we should at least detect it.

Cc: "12.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>