drm/amd/pm: add support to umd P-state "fetch" function for vangogh

This patch is to add supoort to umd P-state function for vangogh.
It enables the "fetch" function of 3 sysfs nodes: pp_dpm_mclk,
pp_dpm_fclk, pp_dpm_socclk,the function is used to fetch
the current frequency of memclk/fclk/socclk.

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>