drm/amd/display: Skip unnecessary abm disable

[Why]
We try to disable abm immediately when it's being gradually
disabled, which causes backlight ramping being paused in the
middle

[How]
Don't disable abm immediately if abm has been or is being
disabled through set_abm_level path

Reviewed-by: Anthony Koo <anthony.koo@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Sherry Wang <yao.wang1@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>