drm/amd/display: Fix 64-bit division for 32-bit builds

[Why]
32-bit builds break when doing 64-bit division directly.

[How]
Use the div_u64() function instead to perform the division.

Fixes: https://lists.freedesktop.org/archives/dri-devel/2018-December/201008.html


Signed-off-by: Ken Chalmers <ken.chalmers@amd.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>