drm/amd/display: Power down front end in init_hw.

front end is initialized during init_hw, but not
power gated. There are some left over valuse and will
cause some diags test failed. Power gated all front
end pipes will make sure every test has same starting
point.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>