drm/panthor: Track the number of mmap on a BO

drm/panthor: Track the number of mmap on a BO

This will be used to order things by reclaimability.

v2:
- Fix refcounting

v3:
- Fix refcounting (again)

v4:
- Collect R-b

v5:
- Collect R-b

v6:
- Warn on is_imported() (not supposed to happen since mmap is
  redirected to the dmabuf layer) instead of adding a conditional
  in panthor_gem_vm_open()

v7:
- No changes

Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patch.msgid.link/20260401134854.2275433-10-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>