drm/panthor: Add GEM logical block

Anything relating to GEM object management is placed here. Nothing
particularly interesting here, given the implementation is based on
drm_gem_shmem_object, which is doing most of the work.

v4:
- Force kernel BOs to be GPU mapped
- Make panthor_kernel_bo_destroy() robust against ERR/NULL BO pointers
  to simplify the call sites

v3:
- Add acks for the MIT/GPL2 relicensing
- Provide a panthor_kernel_bo abstraction for buffer objects managed by
  the kernel (will replace panthor_fw_mem and be used everywhere we were
  using panthor_gem_create_and_map() before)
- Adjust things to match drm_gpuvm changes
- Change return of panthor_gem_create_with_handle() to int

Co-developed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Steven Price <steven.price@arm.com> # MIT+GPL2 relicensing,Arm
Acked-by: Grant Likely <grant.likely@linaro.org> # MIT+GPL2 relicensing,Linaro
Acked-by: Boris Brezillon <boris.brezillon@collabora.com> # MIT+GPL2 relicensing,Collabora