As we give page directory pointer (lvl 3) structure
for pte insertion, we can fold both versions into
one function by teaching it to get pdp regardless
of top level.

v2: naming and asserts (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190816094754.26492-1-mika.kuoppala@linux.intel.com