drm/amd/pm: optimize AC timing programming

Programming AC Timing Parameters is only dependent on MCLK.
No need to nest loop for each SCLK DPM level.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>