drm/amd/pm: correct the way for retrieving current uclk frequency

There is some problem with average frequency reading for now. So,
we switch to the target frequency reading.

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