drm/amdgpu/pm: document pp_dpm_sclk pp_dpm_mclk pp_dpm_pcie (v2)

Used for manually masking dpm states.

v2: drop comment about current state (Rex)

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