video: fbdev: valkyriefb.c: fix warning comparing pointer to 0

Fix the following coccicheck warning:

drivers/video/fbdev/valkyriefb.c:348:10-11: WARNING comparing pointer to
0, suggest !E
drivers/video/fbdev/valkyriefb.c:334:12-13: WARNING comparing pointer to
0
drivers/video/fbdev/valkyriefb.c:348:10-11: WARNING comparing pointer to
0

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200429140942.8137-1-yanaijie@huawei.com