drm/i915/gtt: Recursive ppgtt clear for gen8

With an explicit level, we can refactor the separate clear functions
as a simple recursive function. The additional knowledge of the level
allows us to spot when we can free an entire subtree at once.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190712112725.2892-3-chris@chris-wilson.co.uk