drm/radeon/kms: fix tv module parameter

The tv parameter was added to disable the tv-out connector,
however, it caused a crash if it was set to 0 due to
drm_connector_init not getting called.  If tv=0, don't
attempt to add the connector.

Might fix:
https://bugzilla.kernel.org/show_bug.cgi?id=17241



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