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

Implemented here for DCE 6,8,10,11.
Successfully tested on DCE 10 with AMD R9 380 Tonga.
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: NMario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org