drm/i915: Reorganize intel_enable_ppgtt

This patch consolidates the way in which we handle the various supported
PPGTT by module parameter in addition to what the hardware supports. It
strives to make doing the right thing in the code as simple as possible,
with the USES_ macros.

I've opted to add the full PPGTT argument simply so one can see how I
intend to use this function. It will not/cannot be used until later.

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