drm/i915/gtt: Store scratch page size alongside not in the common struct

As the scratch page is the only one to be allocated with variable size,
rather than keep an unused slot in all i915_page_table structs, store it
alongside the vm->scratch_page.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190305135430.4948-1-chris@chris-wilson.co.uk