drm/panthor: Part ways with drm_gem_shmem_object

While drm_gem_shmem_object does most of the job we need it to do, the
way sub-resources (pages, sgt, vmap) are handled and their lifetimes
gets in the way of BO reclaim. There has been attempts to address
that [1], but in the meantime, new gem_shmem users were introduced
(accel drivers), and some of them manually free some of these resources.
This makes things harder to control/sanitize/validate.

Thomas Zimmerman is not a huge fan of enforcing lifetimes of sub-resources
and forcing gem_shmem users to go through new gem_shmem helpers when they
need manual control of some sort, and I believe this is a dead end if
we don't force users to follow some stricter rules through carefully
designed helpers, because there will always be one user doing crazy things
with gem_shmem_object internals, which ends up tripping out the common
helpers when they are called.

The consensus we reached was that we would be better off forking
gem_shmem in panthor. So here we are, parting ways with gem_shmem. The
current transition tries to minimize the changes, but there are still
some aspects that are different, the main one being that we no longer
have a pages_use_count, and pages stays around until the GEM object is
destroyed (or when evicted once we've added a shrinker). The sgt also
no longer retains pages. This is losely based on how msm does things by
the way.

If there's any interest in sharing code (probably with msm, since the
panthor shrinker is going to be losely based on the msm implementation),
we can always change gears and do that once we have everything
working/merged.

[1]https://patchwork.kernel.org/project/dri-devel/patch/20240105184624.508603-1-dmitry.osipenko@collabora.com/

v2:
- Fix refcounting
- Add a _locked suffix to a bunch of functions expecting the resv lock
  to be held
- Take the lock before releasing resources in panthor_gem_free_object()

v3:
- Use ERR_CAST() to fix an ERR-ptr deref
- Add missing resv_[un]lock() around a panthor_gem_backing_unpin_locked()
  call

v4:
- Fix an error path in panthor_gem_vmap_get_locked()
- Don't leave bo->base.pages with an ERR_PTR()
- Make panthor_gem_{pin,unpin}[_locked]() more consistent
- Don't fail in panthor_gem_dev_map_get_sgt_locked() if the pages are not
  allocated

v5:
- Add missing static specifier on our vm_ops

v6:
- Fix huge_fault handling
- s/uint32_t/u32/
- s/drm_dbg_kms/drm_dbg_driver/
- Collect R-bs

v7:
- No changes

Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patch.msgid.link/20260401134854.2275433-7-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>

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