drm/i915/selftests: Fixup missing MI_MEM_VIRTUAL for live_hangcheck

We always want to use a virtual address (i.e. use the GTT) for
MI_STORE_DWORD_IMM, but forgot the ever so important flag in
live_hangcheck for gen3.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180706142323.25699-2-chris@chris-wilson.co.uk