weston: fix fbdev backend.

The compilation of the program weston-launcher is currently disabled, so
the only way to launch Weston (on framebuffer) is to execute (as root):

  $ openvt -c 2 -s -- weston \
  	--backend=/usr/lib/weston/fbdev-backend.so \
	--tty=2 \
	--log=/tmp/weston.log

Unfortunately you will only end up with a black screen. The reason is
given in the log file:

  /dev/tty2 is already in graphics mode, is another display server running?

Here is the fix from upcoming version 1.4.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>