drm/i915/gtt: Fix rounding for 36b

The top-level page directory for 36b is a single entry, not multiple
like 32b. Fix up the rounding on the calculation of the size of the top
level so that we populate the 4th level correctly for 36b.

Reported-by: Jose Souza <jose.souza@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: 1eda701e ("drm/i915/gtt: Recursive cleanup for gen8")
Cc: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Cc: Jose Souza <jose.souza@intel.com>
Tested-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190719130737.5835-1-chris@chris-wilson.co.uk