drm/amd/display: change hw sequence

[Why]
t9 delay func is called twice after setting power off, the unexpected
action results in a doubling of the added 140ms t9 delay

[How]
If the backlight has been turned off, does not turn it off again

Signed-off-by: Sherry <Yao.Wang1@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>