drm/radeon/dpm/rs780: fix force_performance state for same sclks

If the low and high sclks within a power state are the same,
there no need to enable sclk scaling.  Enabling sclk scaling
can cause display stability issues on some boards.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>