drm/amd/pm: remove the "set" function of pp_dpm_mclk for vangogh

This patch is to remove the "set" function of pp_dpm_mclk for vangogh.
For vangogh, mclk bonds with fclk, they will lock each other
on the same perfomance level. But according to the smu message from pmfw,
only fclk is allowed to set value manually, so remove the unnecessary
code of "set" function for mclk.

Signed-off-by: Xiaojian Du <Xiaojian.Du@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>