drm/amdgpu: clear job on failure in amdgpu_job_alloc(_with_ib)

drm/amdgpu: clear job on failure in amdgpu_job_alloc(_with_ib)

If memory is freed we need to nullify the pointer or the caller
might call kfree again (eg: amdgpu_cs_parser_fini calls kfree on
all non-null job pointers).

Fixes: db36632ea51e ("drm/amdgpu: clean up and unify hw fence handling")
Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>