drm/amd/display: convert to use le16_add_cpu()

Convert cpu_to_le16(le16_to_cpu(E1) + E2) to use le16_add_cpu().

Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>