drm/amd/display: refactor vupdate interrupt registration

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>