fbdev: omapfb: constify ctrl_caps, color_caps, panel_attr_grp and ctrl_attr_grp

File size before:
   text	   data	    bss	    dec	    hex	filename
  13007	   1536	    156	  14699	   396b	drivers/video/fbdev/omap/omapfb_main.o

File size after adding 'const':
   text	   data	    bss	    dec	    hex	filename
  13135	   1408	    156	  14699	   396b	drivers/video/fbdev/omap/omapfb_main.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>