amd/powerplay: implement the vega12_force_clock_level interface

pp_dpm_sclk/pp_dpm_mclk in sysfs implemented to force
gfxclk/uclk dpm level for Vega12

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