m68knommu: fix 'screen_bits' defined but not used

The 'screen_bits' LCD initializer bit array is only required if
CONFIG_INIT_LCD is enabled, otherwise we get the following warning:

    arch/m68k/68000/screen.h:4:22: warning: 'screen_bits' defined but not used [-Wunused-variable]
     static unsigned char screen_bits[] = {
                          ^~~~~~~~~~~

Make the inclusion of 'screen_bits' conditional on CONFIG_INIT_LCD.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>
Reported-by: kernel test robot <lkp@intel.com>