Don't dereference "db->dsi_dev" when it is an error pointer.

Fixes: 249a4f5e ("drm/panel: Add Magnachip D53E6EA8966 Panel Driver")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/de0273a8-8910-4ac4-b4ed-f7691c4d2ca6@kili.mountain