In our swizzling selftests, we cannot predict the physical address of
the target page (at least not simply!) and so skip bit17 swizzles.
However, there are two bit17 swizzle modes and we only skipped one, with
the second being observed on the lab gdg causing the test to fail,
as soon as we hit a page with bit17 set in its address.

Testcase: igt/drv_selftest/live_objects #gdg
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180709194915.5789-1-chris@chris-wilson.co.uk