r600g/sb: fix stack size computation on evergreen

r600g/sb: fix stack size computation on evergreen

On evergreen we have to reserve 1 stack element in some additional cases
besides the ones mentioned in the docs, but stack size computation was
recently reimplemented exactly as described in the docs by the patch that
added workarounds for stack issues on EG/CM, resulting in regressions
with some apps (Serious Sam 3).

This patch fixes it by restoring previous behavior.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=72369

Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>
Cc: "10.0" <mesa-stable@lists.freedesktop.org>
Tested-by: Andre Heider <a.heider@gmail.com>