drm/amdgpu: Retain job->vm in amdgpu_job_prepare_job

The field job->vm is used in function amdgpu_job_run to get the page
table re-generation counter and decide whether the job should be skipped.

Specifically, function amdgpu_vm_generation checks if the VM is valid for this job to use.
For instance, if a gfx job depends on a cancelled sdma job from entity vm->delayed,
then the gfx job should be skipped.

Fixes: 26c95e83 ("drm/amdgpu: set the VM pointer to NULL in amdgpu_job_prepare")
Signed-off-by: YuanShang <YuanShang.Mao@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>