[WHY]
Currently we don't wait for blacklight programming completion in DMCU
when setting backlight level. Some sequences such as PSR static screen
event trigger reprogramming requires it to be complete.

[How]
Add generic wait for dmcu command completion in set backlight level.
Signed-off-by: NSivapiriyanKumarasamy <sivapiriyan.kumarasamy@amd.com>
Reviewed-by: NAnthony Koo <Anthony.Koo@amd.com>
Acked-by: NLeo Li <sunpeng.li@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>