drm/omap: constify attribute_group structures.

drm/omap: constify attribute_group structures.

attribute_groups are not supposed to change at runtime. All functions
working with attribute_groups provided by <linux/sysfs.h> work with const
attribute_group. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
   2673	    368	      0	   3041	    be1	panel-tpo-td043mtea1.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   2721	    304	      0	   3025	    bd1	panel-tpo-td043mtea1.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>