drm/panthor: Keep a ref to the VM at the panthor_kernel_bo level

Avoids use-after-free situations when panthor_fw_unplug() is called
and the kernel BO was mapped to the FW VM.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240502183813.1612017-3-boris.brezillon@collabora.com

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.

v6:
- Add Maxime's and Heiko's acks
- Return a page-aligned BO size to userspace when creating a BO
- Keep header inclusion alphabetically ordered

v5:
- Add Liviu's and Steve's R-b

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
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240229162230.2634044-6-boris.brezillon@collabora.com