video: udlfb: use true,false for bool variables

Fix the following coccicheck warning:

drivers/video/fbdev/udlfb.c:67:12-19: WARNING: Assignment of 0/1 to bool
variable
drivers/video/fbdev/udlfb.c:68:12-20: WARNING: Assignment of 0/1 to bool
variable
drivers/video/fbdev/udlfb.c:69:12-18: WARNING: Assignment of 0/1 to bool
variable

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200422071836.49123-1-yanaijie@huawei.com