drm/i915/selftests: Use igt_random_offset()

Switch igt_vm_isolation() to using igt_random_offset().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200312154708.1720-1-chris@chris-wilson.co.uk