drm/amdgpu: disable CRTCs before teardown

Some code called by drm_crtc_force_disable_all() wants to wait for all
fences, so only do fence teardown after CRTCs are disabled.

Fixes: 84b89bdc ("drm/amdgpu: Turn off CRTCs on driver unload")
Cc: stable@vger.kernel.org # v4.8+
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>