Allocate all page directory variants with alloc_pd. As
the lvl3 and lvl4 variants differ in manipulation, we
need to check for existence of backing phys page before accessing
it.

v2: use err in returns
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190614164350.30415-5-mika.kuoppala@linux.intel.com