drm/amd/display: Keep current gain when ABM disable immediately

[Why]
When system enters s3/s0i3, backlight PWM would set user level.

[How]
ABM disable function add keep current gain to avoid it.

Signed-off-by: Brandon Syu <Brandon.Syu@amd.com>
Reviewed-by: Josip Pavic <Josip.Pavic@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>