drm/radeon: fix audio dto calculation on DCE3+ (v3)

Need to set the wallclock ratio and adjust the phase
and module registers appropriately.  May fix problems
with audio timing at certain display timings.

v2: properly handle clocks below 24mhz
v3: rebase r600 changes

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