drm/amdgpu: add missing comment for the new argument

drm/amdgpu: add missing comment for the new argument

In function 'amdgpu_vm_lock_done_list' update the comment
for the new argument 'vm'.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202509180211.UAqME0zj-lkp@intel.com/
Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>