drm/radeon: Avoid overflows/divide-by-zero in latency_watermark calculations.

drm/radeon: Avoid overflows/divide-by-zero in latency_watermark calculations.

At dot clocks > approx. 250 Mhz, some of these calcs will overflow and
cause miscalculation of latency watermarks, and for some overflows also
divide-by-zero driver crash. Make calcs more overflow resistant.

This is a direct port of the corresponding patch from amdgpu-kms,
copy-paste for cik from dce-8 and si from dce-6, with a slightly
simpler variant for evergreen dce-4/5.

Only tested on DCE-4 evergreen with a Radeon HD-5770.

Signed-off-by: Mario Kleiner <[email protected]>
Cc: Alex Deucher <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>