drm/i915/selftests: Improve handling of iomem around stolen

Use memset_io() on the iomem, and silence sparse as we copy from the
iomem to normal system pages.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210106123939.18435-2-chris@chris-wilson.co.uk