drm/amd/powerplay: support retrieving and adjusting dcefclock power levels V2

User can use "pp_dpm_dcefclk" to retrieve and adjust dcefclock power
levels.

V2: expose this interface for Vega10 and later ASICs only

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