drm/panthor: Fix potential memleak of vma structure

drm/panthor: Fix potential memleak of vma structure

This commit addresses a memleak issue of panthor_vma (or drm_gpuva)
structure in Panthor driver, that can happen if the GPU page table
update operation to map the pages fail.
The issue is very unlikely to occur in practice.

v2: Add panthor_vm_op_ctx_return_vma() helper (Boris)

v3: Add WARN_ON_ONCE (Boris)

Fixes: 647810ec2476 ("drm/panthor: Add the MMU/VM logical block")
Signed-off-by: Akash Goel <akash.goel@arm.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patch.msgid.link/20251021081042.1377406-1-akash.goel@arm.com