drm/drm_gem.c: Remove surplus else after return

else is not generally useful after return

Signed-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230622101857.2298773-1-suijingfeng@loongson.cn