backlight: aw99703 use exponential curve from kp to avoid flickering when changing light environment. Change-Id: If01d5ebda7c2a7bf276387f34ad48a00a9163057 Reviewed-on: https://gerrit.mot.com/2543541 SME-Granted: SME Approvals Granted SLTApproved: Slta Waiver Tested-by: Jira Key Reviewed-by: Yeqing Wang <wangyq13@motorola.com> Reviewed-by: Huosheng Liao <liaohs@motorola.com> Submit-Approved: Jira Key