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

There is some problem with average frequency reading. Thus, we
switch to the target frequency reading instead.

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