omapfb: panel-dsi-cm: constify dsicm_attr_group

File size before:
   text	   data	    bss	    dec	    hex	filename
   4657	    464	      0	   5121	   1401	drivers/video/fbdev/omap2/omapfb/displays/panel-dsi-cm.o

File size after adding 'const':
   text	   data	    bss	    dec	    hex	filename
   4713	    400	      0	   5113	   13f9	drivers/video/fbdev/omap2/omapfb/displays/panel-dsi-cm.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>