backlight: mbp_nvidia_bl - Add a debug switch

This patch adds a debug switch to enable (little) diagnostic
output, to help to trace down the remaining problems.

Signed-off-by: Mario Schwalbe <schwalbe@inf.tu-dresden.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>