drm/i915/gtt: Account for preallocation in asserts

Our asserts allow for the PDEs to be allocated concurrently, but we did
not account for the aliasing-ppgtt to be preallocated on top.

Testcase: igt/gem_ppgtt #bsw
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191207221453.2802627-1-chris@chris-wilson.co.uk