drm/amd/display: hide VGH asic specific structs

The pmfw structs are specific to the asic and should not be
present in base clk_mgr struct

v2: squash in SI fix (Alex)

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>