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