drm/amd/display: put back front end initialization sequence

[Why]
Seamless boot optimization removed proper front end power off sequence.
In driver disable enable case, this causes driver to power gate hubp
and dpp while there is still memory fetching going on, this can cause
invalid memory requests to be generated which will hang data fabric.

[How]
Put back proper front end power off sequence

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Acked-by: Tony Cheng <Tony.Cheng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>