drm/i915/selftests: Exercise CS TLB invalidation

Check that we are correctly invalidating the TLB at the start of a
batch after updating the GTT.

v2: Comments and hold the request reference while spinning

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190919131414.7495-1-chris@chris-wilson.co.uk