c6x: kernel: setup: Add screen_info global variable

Or can not pass building with allmodconfig:

    LD      init/built-in.o
  drivers/built-in.o: In function `vgacon_switch':
  vgacon.c:(.text+0x47f8): undefined reference to `screen_info'
  vgacon.c:(.text+0x4810): undefined reference to `screen_info'
  drivers/built-in.o: In function `vgacon_resize':
  vgacon.c:(.text+0x4ac8): undefined reference to `screen_info'
  vgacon.c:(.text+0x4acc): undefined reference to `screen_info'
  drivers/built-in.o: In function `vgacon_save_screen':
  vgacon.c:(.text+0x4cc8): undefined reference to `screen_info'
  drivers/built-in.o:vgacon.c:(.text+0x4cd0): more undefined references to `screen_info' follow

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Mark Salter <msalter@redhat.com>