drm/radeon: use sw CTS/N values for audio on DCE4+

Use the driver calculated CTS and N values rather than
having hardware generate them.  This allows us to use
the modeline pixel clock rather than the actual pll clock
when setting up the dto for audio.  Fixes problems with
audio playback rate on certain asics if the pll clock
does not match the pixel clock exactly.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>