drm/amd/pp: Refine pp_dpm_force_clock_level functions

Only when user set manual performance mode, driver
enable pp_dpm_force_clock_level.
so check the mode in pp_dpm_force_clock_level,
and delete the same logic in callback functions.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>