drm/amd/display: fix duplicate aux command with AMD aux backlight

drm/amd/display: fix duplicate aux command with AMD aux backlight

when using AMD aux backlight control, we avoid sending backlight
update commands to DMUB firmware because it is controlled by aux commands
in driver.

Reviewed-by: Iswara Nagulendran <iswara.nagulendran@amd.com>
Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Signed-off-by: Harry VanZyllDeJong <hvanzyll@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>