drm/amdgpu: fix error handling in amdgpu_copy_buffer

drm/amdgpu: fix error handling in amdgpu_copy_buffer

drm_sched_job_add_resv_dependencies can fail in amdgpu_ttm_prepare_job.
In this case we need to use amdgpu_job_free to release memory.

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v4: moved job pointer clearing to a different patchset
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Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>