fbdev: mmp: Constify struct mmp_overlay_ops

'struct mmp_overlay_ops' is not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  11798	    555	     16	  12369	   3051	drivers/video/fbdev/mmp/hw/mmp_ctrl.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  11834	    507	     16	  12357	   3045	drivers/video/fbdev/mmp/hw/mmp_ctrl.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Helge Deller <deller@gmx.de>