drm/gem: Make the GEM LRU lock part of drm_device

drm/gem: Make the GEM LRU lock part of drm_device

Recently, a few races have been discovered in the GEM LRU logic, all
of them caused by the fact the LRU lock is accessed through
gem->lru->lock, and that very same lock also protects changes to
gem->lru, leading to situations where gem->lru needs to first be
accessed without the lock held, to then get the lru to access the lock
through and finally take the lock and do the expected operation.

Currently, the only driver making use of this API (MSM) declares a
device-wide lock, and the user we're about to add (panthor) will
do the same. There's no evidence that we will ever have a driver
that wants different pools of LRUs protected by different locks under
the same drm_device. So we're better off moving this lock to drm_device
and always locking it through obj->dev->gem_lru_mutex, or directly
through dev->gem_lru_mutex.

If anyone ever needs more fine-grained locking, this can be revisited
to pass some drm_gem_lru_pool object representing the pool of LRUs
under a specific lock, but for now, the per-device lock seems to be
enough.

Fixes: e7c2af13f811 ("drm/gem: Add LRU/shrinker helper")
Reported-by: Chia-I Wu <olvaffe@gmail.com>
Closes: https://gitlab.freedesktop.org/panfrost/linux/-/work_items/86
Reviewed-by: Rob Clark <rob.clark@oss.qualcomm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: https://patch.msgid.link/20260518-panthor-shrinker-fixes-v4-1-1920234470d5@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>