drm/radeon: avoid turning off spread spectrum for used pll

If spread spectrum is enabled and in use for a given pll we
should not turn it off as it will lead to turning off display
for crtc that use the pll (this behavior was observed on chelsea
edp).

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