drm/amdgpu: Fix use-after-free in amdgpu_cs_ioctl

[Why]
In amdgpu_cs_ioctl, amdgpu_job_free could be performed ealier if there
is -ERESTARTSYS error. In this case, job->hw_fence could be not
initialized yet. Putting hw_fence during amdgpu_job_free could lead to a
use-after-free warning.

[How]
Check if drm_sched_job_init is performed before job_free by checking
s_fence.

v2: Check hw_fence.ops instead since it could be NULL if fence is not
initialized. Reverse the condition since !=NULL check is discouraged in
kernel.

Signed-off-by: YuBiao Wang <YuBiao.Wang@amd.com>
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>