fbdev: mb862xxfb: Fix defined but not used error

socrates_gc_mode is defined at the top-level but then only used inside
an #ifdef CONFIG_FB_MB862XX_LIME, leading to an error with some configs:

  drivers/video/fbdev/mb862xx/mb862xxfbdrv.c:36:31: error: ‘socrates_gc_mode’ defined but not used
     36 | static struct mb862xx_gc_mode socrates_gc_mode = {

Fix it by moving socrates_gc_mode inside that ifdef, immediately prior
to the only function where it's used.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Helge Deller <deller@gmx.de>