drm/i915/gtt: Fold gen8 insertions into one

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