video: fbdev: fsl-diu-fb: remove unneeded variable 'res'

Eliminate the following coccicheck warning:

drivers/video/fbdev/fsl-diu-fb.c:1428:5-8: Unneeded variable: "res".
Return "0" on line 1450

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Acked-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200910140558.1191423-1-yanaijie@huawei.com