drm/i915/selftests: fix subtraction overflow bug

drm/i915/selftests: fix subtraction overflow bug

On some machines hole_end can be small enough to cause subtraction
overflow. On the other side (addr + 2 * min_alignment) can overflow
in case of mock tests. This patch should handle both cases.

Fixes: e1c5f754067b59 ("drm/i915: Avoid overflow in computing pot_hole loop termination")
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/3674
Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220624113528.2159210-1-andrzej.hajda@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>