drm/radeon/dpm: fix 120hz handling harder

Need to expand the check to handle short circuiting
if the selected state is the same as current state.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=87796



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