drm/i915: Make GEM contexts track DRM clients

Make GEM contexts keep a reference to i915_drm_client for the whole of
of their lifetime which will come handy in following patches.

v2: Don't bother supporting selftests contexts from debugfs. (Chris)
v3 (Lucas): Finish constructing ctx before adding it to the list
v4 (Ram): Rebase.
v5: Trivial rebase for proto ctx changes.
v6: Rebase after clients no longer track name and pid.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> # v5
Reviewed-by: Aravind Iddamsetty <aravind.iddamsetty@intel.com> # v5
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220401142205.3123159-3-tvrtko.ursulin@linux.intel.com