drm/i915/selftests: fix subtraction overflow bug
In the Linux kernel, the following vulnerability has been resolved:

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.

(cherry picked from commit ab3edc679c552a466e4bf0b11af3666008bd65a2)