drm/i915/gtt: Recursive cleanup for gen8

With an explicit level, we can refactor the separate cleanup functions
as a simple recursive function. We take the opportunity to pass down the
size of each level so that we can deal with the different sizes of
top-level and avoid over allocating for 32/36-bit vm.

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/20190712112725.2892-2-chris@chris-wilson.co.uk