add vega20_find_highest_dpm_level, vega20_find_lowest_dpm_level,
    vega20_force_highest_dpm and vega20_force_lowest_dpm functions
    to support sys interface for SMU11.

v2: fix highest/lowest implementation changes error.
Signed-off-by: Chengming Gui <Jack.Gui@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>