drm/i915/gtt: Convert vm->scratch into an array

Each level has its own scratch. Make the levels more obvious by forgoing
the fancy similarly names and replace them with a number. 0 is the bottom
most level, the physical page used for actual data; 1+ are the page
directories.

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/20190712094327.24437-5-chris@chris-wilson.co.uk