drm/i915: Per-process stats work better when evaluated per-process

The idea of printing objects used by each process is to judge how each
process is using them. This means that we need to evaluate whether the
object is bound for that particular process, rather than just whether it
is bound into the global GTT.

v2: Restore the non-full-ppgtt path for simplicity as we may not even
    create vma with older hardware.

v3: Tweak handling of global entries and default context entries.

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