drm/radeon/kms: disable the tv encoder when tv/cv is not in use

Switching between TV and VGA caused VGA to break on some systems
since the TV encoder was left enabled when VGA was used.

fixes fdo bug 25520.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>