drm/amd/display: fix 64bit division issue on 32bit OS

Replace "/" with div_u64 for 64bit division on 32bit OS.

Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>