drm/amd/display: Refactor disable front end pipes.

There are different code to disable front end, it is
difficult to debug and adding new process.
This refactor makes all disable front end call the same
functions.

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>