drm/amd/display: Use div_u64 for flip timestamp ns to ms

Resolves __udivdi3 missing errors when building for i386.

Fixes: 6378ef012ddc ("drm/amd/display: Add below the range support for FreeSync")

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>