drm/amdgpu/dce11: add missing drm_mode_config_cleanup call

All other amdgpu/dce_v* files have this call, it's only mysteriously
missing from dce_v11_0.c since the file was added and causes leaks.

Fixes: aaa36a97 ("drm/amdgpu: Add initial VI support")
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org