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