nvidiafb: Enable debugging messages a Kconfig option

Let the user enable debugging messages in nvidiafb.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>