staging: xgifb: make const array static to shink object code size

Don't populate const array LCDARefreshIndex on the stack, instead
make it static.  Makes the object code smaller by 340 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  84949	  12336	      0	  97285	  17c05	drivers/staging/xgifb/vb_setmode.o

After:
   text	   data	    bss	    dec	    hex	filename
  84506	  12432	      0	  96938	  17aaa	drivers/staging/xgifb/vb_setmode.o

(gcc version 7.2.0 x86_64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>