drm/i915: Abstract PPGTT enabling

Since we've already set up a nice vtable to abstract other PPGTT
functions, also abstract the actual register programming to enable
things.

This function will probably need to change a bit as we implement real
processes.

v2: Resolve conflicts due to patch series reordering.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>