drm/amd/display: After program backend, also program front end regs.

Issue:
In case of two 4K@60 + one non-4k monitor, when unplug non-4k monitor,
the remain two 4k monitor don't work properly.
Reason:
In that case, two 4k use two pipes and no split, when unplug happens,
those two monitor will use 4 pipes and split, but on that time, frontend
is not programed properly.
Solution:
After programed backend, front end should be programmed as per new pipe
setting.

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