drm/i915/sdvo: Fix up debug output to not split lines

It leads to a big mess when stuff interleaves. Especially with the new
patch I've submitted for the drm core to no longer artificially split
up debug messages.

v2: The size parameter to snprintf includes the terminating 0, but the
return value does not. Adjust the logic accordingly. Spotted by Mika.

Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>