drm/i915/gtt: Record the scratch pte

Record the scratch PTE encoding upon creation rather than recomputing
the bits everytime. This is important for the next patch where we forgo
having a valid scratch page with which we may compute the bits and so
require keeping the PTE value instead.

v2: Fix up scrub_64K to use scratch_pte as well.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181029182721.29568-1-chris@chris-wilson.co.uk