drm/i915: Create an init vm

Move all the similar address space (VM) initialization code to one
function. Until we have multiple VMs, there should only ever be 1 VM.
The aliasing ppgtt is a special case without it's own VM (since it
doesn't need it's own address space management).

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