drm/i915: vfuncs for ppgtt

Like for the global gtt we want a notch more flexibility here. Only
big change (besides a few tiny function parameter adjustments) was to
move gen6_ppgtt_insert_entries up (and remove _sg_ from its name, we
only have one kind of insert_entries since the last gtt cleanup).

We could also extract the platform ppgtt setup/teardown code a bit
better, but I don't care that much.

With this we have the hw details of pte writing nicely hidden away
behind a bit of abstraction. Which should pave the way for
different/multiple ppgtts (e.g. what we need for real ppgtt support).

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>