drm/amdgpu: fix error handling in amdgpu_cs_user_fence_chunk

Slowly leaking memory one page at a time :)

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>