drm/amd/display:: Fix NULL pointer in Raven hotplug

Programming sequence to frontend and backend has been switched.
In such case, program_scaler() is getting called when programming
frontend, and should be removed from backend programming routine.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Jerry (Fangzhi) Zuo <Jerry.Zuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>