drm/i915: Differentiate the aliasing_ppgtt with an invalid filp

Use an invalid filp so that the aliasing_ppgtt can be clearly
identified.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170215084357.19977-22-chris@chris-wilson.co.uk