drm/radeon: use 64-bit math to calculate CTS values for audio (v2)

commit 062c2e4363451d49ef840232fe65e8bff0dde2a5 upstream.

Avoid losing precision.  See bug:
https://bugs.freedesktop.org/show_bug.cgi?id=69675



v2: fix math as per Anssi's comments.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>