modetest: fix Segmentation fault

If use -P option without -s option, the program segfaults due to
dev.mode.bo being NULL.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
[Emil Velikov: Tweak the commit message.]
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>