Force DVI mode if the user specifies radeon.audio=0.  The driver
doesn't handle HDMI mode properly in some cases.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=27731Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>