drm/i915/gem: Remove per-client stats from debugfs/i915_gem_objects

Rather than trying to avoid the use-after-free possible with the current
context chasing, simply remove presentation of the per-client stats from
debugfs. While we know from bug reports that this debugfs/i915_gem_objects
has been used by chromeos (and chrome itself) for debug purposes, google
suggests that it is unparsed, so we are free to invoke debugfs is not
ABI and remove details from it.

Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210118110854.1873-1-chris@chris-wilson.co.uk