drm/panel: mantix: Don't dereference NULL mode

Don't dereference mode which was just NULL checked.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/659158549f3c6cc1c71ceed0943e760e861c1206.1602584953.git.agx@sigxcpu.org