drm/radeon/dpm: disable ss on Cayman

Spread spectrum seems to cause hangs when dynamic clock
switching is enabled.  Disable it for now. This does not
affect performance or the amount of power saved. Tracked
down by Martin Andersson.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=69723



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