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

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

BugLink: http://bugs.launchpad.net/bugs/1694621
commit e190ed1ea7458e446230de4113cc5d53b8dc4ec8 upstream.

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 ("divide error: 0000 [#1] PREEMPT SMP" in
"dce_v10_0_latency_watermark+0x12d/0x190").

This zero-divide happened, e.g., on AMD Tonga Pro under DCE-10,
on a Displayport panel when trying to set a video mode of 2560x1440
at 165 Hz vrefresh with a dot clock of 635.540 Mhz.

Refine calculations to avoid the overflows.

Tested for DCE-10 with R9 380 Tonga + ASUS ROG PG279 panel.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>