video: sunxi: Change sunxi_get_mon_desc() to not return NULL for the default case

When building with gcc 9.2.0, the following build warning was seen:

  drivers/video/sunxi/sunxi_display.c: In function 'video_hw_init':
  drivers/video/sunxi/sunxi_display.c:1217:2:
    error: '%s' directive argument is null [-Werror=format-overflow=]

Change sunxi_get_mon_desc() to not return NULL for the default case,
to fix the compiler warning.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>