drm/i915: Report the number of closed vma held by each context in debugfs

Include the total size of closed vma when reporting the per_ctx_stats of
debugfs/i915_gem_objects.

Whilst adjusting the context tracking, note that we can simply use our
list of contexts in i915->contexts rather than circumlocute via
dev->filelist and the per-file context idr, with the result that we can
show objects allocated to different vm (i.e. contexts within a file).

We change the output to show every context of each client, with its own
unique set of objects (for full-ppgtt machines, i.e. gen7+, for older
hardware all objects are in the global gtt and so can not be associated
with a single context). That should result in no loss of information,
and for gen7+, no duplication of active objects.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190107115509.12523-2-chris@chris-wilson.co.uk