drm/i915/selftests: Exercise rc6 w/a handling

Reading from CTX_INFO upsets rc6, requiring us to detect and prevent
possible rc6 context corruption. Poke at the bear!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Tested-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191119154723.3311814-1-chris@chris-wilson.co.uk