fbdev: mx3fb: remove unused variable 'irq'

Remove the variable 'irq' that is set but never used.

Signed-off-by: Xiaofei Tan <tanxiaofei@huawei.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1600334360-4214-1-git-send-email-tanxiaofei@huawei.com