drm/i915: Split GEN6 PPGTT initialization up

Simply to match the GEN8 style of PPGTT initialization, split up the
allocations and mappings. Unlike GEN8, we skip a separate dma_addr_t
allocation function, as it is much simpler pre-gen8.

With this code it would be easy to make a more general PPGTT
initialization function with per GEN alloc/map/etc. or use a common
helper, similar to the ringbuffer code. I don't see a benefit to doing
this just yet, but who knows...

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>