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

Replace "/" with div_u64 for 32-bit OS. On 32-bit OS,
the use of "/" for 64-bit division will cause build error,
i.e. "__udivdi3/__divdi3 undefined!".

Fixes: ea7154d8 ("drm/amd/display: Update dcn30_apply_idle_power_optimizations() code")
Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>