drm/i915: thread address space through execbuf

This represents the first half of hooking up VMs to execbuf. Here we
basically pass an address space all around to the different internal
functions. It should be much more readable, and have less risk than the
second half, which begins switching over to using VMAs instead of an
obj,vm.

The overall series echoes this style of, "add a VM, then make it smart
later"

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Switch a BUG_ON to WARN_ON.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>