video: fbdev: smscufx: Fix null-ptr-deref in ufx_usb_probe()

I got a null-ptr-deref report:

BUG: kernel NULL pointer dereference, address: 0000000000000000
...
RIP: 0010:fb_destroy_modelist+0x38/0x100
...
Call Trace:
 ufx_usb_probe.cold+0x2b5/0xac1 [smscufx]
 usb_probe_interface+0x1aa/0x3c0 [usbcore]
 really_probe+0x167/0x460
...
 ret_from_fork+0x1f/0x30

If fb_alloc_cmap() fails in ufx_usb_probe(), fb_destroy_modelist() will
be called to destroy modelist in the error handling path. But modelist
has not been initialized yet, so it will result in null-ptr-deref.

Initialize modelist before calling fb_alloc_cmap() to fix this bug.

Fixes: 3c8a63e2

 ("Add support for SMSC UFX6000/7000 USB display adapters")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Helge Deller <deller@gmx.de>