video/udlfb: fix line counting in fb_write

commit b8c4321f3d194469007f5f5f2b34ec278c264a04 upstream.

Line 0 and 1 were both written to line 0 (on the display) and all subsequent
lines had an offset of -1. The result was that the last line on the display
was never overwritten by writes to /dev/fbN.

Signed-off-by: Alexander Holler <holler@ahsoftware.de>
Acked-by: Bernie Thompson <bernie@plugable.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>