fbdev: imxfb: Check fb_add_videomode to prevent null-ptr-deref

In the Linux kernel, the following vulnerability has been resolved:

fbdev: imxfb: Check fb_add_videomode to prevent null-ptr-deref

fb_add_videomode() can fail with -ENOMEM when its internal kmalloc() cannot allocate a struct fb_modelist. If that happens, the modelist stays empty but the driver continues to register. Add a check for its return value to prevent poteintial null-ptr-deref, which is similar to the commit 17186f1f90d3 ("fbdev: Fix do_register_framebuffer to prevent null-ptr-deref in fb_videomode_to_var").