We only need to register once OS calls the interrupt control.
Also, if we are entering static screen mode, disable after ramping is done.
Disable shall be done via timer of 2 seconds regardless of ramping
complete or not, just to simplify.

Also, ramp to mid instead of min, due to better flicker performance...
Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>