drm/i915/selftests: Replace magic 1<<22 with MI_USE_GGTT/MI_MEM_VIRTUAL

Replace the magic bit with the proper symbolic name for instructing
MI_STORE_DWORD_IMM to use a virtual address (on gen3) or the global GTT
address (still virtual!) on gen4+.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180706142323.25699-1-chris@chris-wilson.co.uk
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>