drm/amdgpu: fix ring timeout issue in gfx10 sr-iov environment

commit 26c95e83 ("drm/amdgpu: set the VM pointer to NULL in
amdgpu_job_prepare") set job->vm as NULL if there is no fence. It will
cause emit switch buffer be skippen if job->vm set as NULL.

Check job rather than vm could solve this problem.

Fixes: 26c95e83 ("drm/amdgpu: set the VM pointer to NULL in amdgpu_job_prepare")
Signed-off-by: Lin.Cao <lincao12@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>