drm/i915: Kill obj->gtt_offset

With the getters in place from the previous patch this members serves no
purpose other than saving one spare pointer chase, which will be killed
in the next patch anyway.

Moving to VMAs, this members adds unnecessary confusion since an object
may exist at different offsets in different VMs.

v2: Properly preserve the stolen offset. This code is a bit hacky but it
all goes away when we embed the drm_mm_node and removes the need for the
incorrect patch I submitted previously: "Use gtt_space->start for stolen
reservation"

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