drm/radeon: Make display watermark calculations more accurate

Avoid big roundoff errors in scanline/hactive durations for
high pixel clocks, especially for >= 500 Mhz, and thereby
program more accurate display fifo watermarks.

This is a port of the corresponding amdgpu patch.

Implemented for DCE 4,6,8.
Tested on Evergreen/DCE-4 with Radeon HD-5770.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>