drm/amd/display: Fix compilation warnings on i386

[why]
Compilation error "undefined reference to `__udivdi3'" was
thrown on i386 architecture.

[how]
Use div_u64 for unsigned long division instead of a divide operator.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>