drm/amd/powerplay: add interface for forcing and unforcing dpm limit value

That's base function for forcing and unforcing dpm limit value.

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