drm/i915: Explicitly track DRM clients

Tracking DRM clients more explicitly will allow later patches to
accumulate past and current GPU usage in a centralised place and also
consolidate access to owning task pid/name.

Unique client id is also assigned for the purpose of distinguishing/
consolidating between multiple file descriptors owned by the same process.

v2:
 Chris Wilson:
 * Enclose new members into dedicated structs.
 * Protect against failed sysfs registration.

v3:
 * sysfs_attr_init.

v4:
 * Fix for internal clients.

v5:
 * Use cyclic ida for client id. (Chris)
 * Do not leak pid reference. (Chris)
 * Tidy code with some locals.

v6:
 * Use xa_alloc_cyclic to simplify locking. (Chris)
 * No need to unregister individial sysfs files. (Chris)
 * Rebase on top of fpriv kref.
 * Track client closed status and reflect in sysfs.

v7:
 * Make drm_client more standalone concept.

v8:
 * Simplify sysfs show. (Chris)
 * Always track name and pid.

v9:
 * Fix cyclic id assignment.

v10:
 * No need for a mutex around xa_alloc_cyclic.
 * Refactor sysfs into own function.
 * Unregister sysfs before freeing pid and name.
 * Move clients setup into own function.

v11:
 * Call clients init directly from driver init. (Chris)

v12:
 * Do not fail client add on id wrap. (Maciej)

v13 (Lucas): Rebase.

v14:
 * Dropped sysfs bits.

v15:
 * Dropped tracking of pid/ and name.
 * Dropped RCU freeing of the client object.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> # v11
Reviewed-by: Aravind Iddamsetty <aravind.iddamsetty@intel.com> # v11
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/20220401142205.3123159-2-tvrtko.ursulin@linux.intel.com