drm/amdgpu/pm: move additional logic into amdgpu_dpm_force_performance_level

This is part of the forced performance level.  Move it from
the sysfs handler into amdgpu_dpm_force_performance_level.

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