drm/radeon/cik: stop the sdma engines in the enable() function

We always stop the rings when disabling the engines so just
call the stop functions directly from the sdma enable function.
This way the rings' status is set correctly on suspend so
there are no problems on resume.  Fixes resume failures that
result in acceleration getting disabled.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org