drm/amd/pm: enable the "fetch" function of pp_dpm_vclk/dclk for vangogh

This patch is to enable the "fetch" function of pp_dpm_vclk and
pp_dpm_dclk.
It allows to fetch the current frequency of vcn and dcn and their
DPM levels for vangogh.

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