drm/amd/display: Change to correct unit on audio rate

[Why]
Formula uses kHz in their formula while our driver operates with Hz.

[How]
Divide audio rate by 1000 on the initial variable that is entered into formula.

Signed-off-by: Chris Park <Chris.Park@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Acked-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>