drm/radeon/dpm/rs780: don't enable sclk scaling if not required

If the low and high sclks are the same, there is no need to
enable sclk scaling.  This causes display stability issues on
certain boards.

Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=60857



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