drm/amd/display: fix 64 bit divide in freesync code

Use div_u64() rather than a a 64 bit divide.

Fixes: 3fe5739d ("drm/amd/display: Add flip interval workaround")
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Angus Wang <Angus.Wang@amd.com>
Cc: Anthony Koo <Anthony.Koo@amd.com>
Cc: Aric Cyr <Aric.Cyr@amd.com>
Cc: Nathan Chancellor <nathan@kernel.org>