drm/amd/display: Fix issue breaking 32bit Linux build

[Why]
Currently, the 32bit linux build is failing due to an issue with using the
built-in / operator with a 64bit dividend. Doing so generates code which
calls __udivdi3() in libgcc. However, libgcc is not linked with the kernel
at this point in the build, hence this causes the 32bit build to fail to
compile.

[How]
Change the / operator to div_u64 instead.

Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Pavle Kotarac <Pavle.Kotarac@amd.com>
Signed-off-by: Hayden Goodfellow <Hayden.Goodfellow@amd.com>