drm/i915: Provide PDP updates via MMIO

The initial implementation of this function used MMIO to write the PDPs.
Upon review it was determined (correctly) that the docs say to use LRI.
The issue is there are times where we want to do a synchronous write
(GPU reset).

I've tested this, and it works. I've verified with as many people as
possible that it should work.

This should fix the failing reset problems.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>