drm/i915/bdw: Initialize the PDEs

The upcoming clear and insert routines will expect that PDEs all point
to valid Page Directories. Doing that lazily doesn't really buy us
anything.

The page allocation is done regardless earlier in init so it shouldn't
hurt set the PDEs.

v2: Squash in patches to implement fixed PDE write function:

- If I had done this in the first place, the bug that's going to be
  fixed in an upcoming patch would have been much easier to find.

- Use WB for PDEs.

  The PAT bit is used for page size. 2ME PDEs aren't even supported in
  BDW, so this was completely invalid. The solution is to make our
  PDEs WB+LLC instead of the pervious WB+eLLC. As far as I can guess,
  this change won't matter for performance.

  Thanks to Ville for the quick correction when discussing on IRC.

v3: Return the pde type for pde encoding (Damien)

Reviewed-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>