drm/i915/gtt: Use NULL to encode scratch shadow entries

We can simplify our gtt walking code by comparing against NULL for
scratch entries as opposed to looking up the distinct per-level scratch
pointer.

The only caveat is to remember to protect external parties and map the
NULL to the scratch top pd.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190712094327.24437-6-chris@chris-wilson.co.uk