drm: i915: sysfs: 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
   4028	   1088	      0	   5116	   13fc	drivers/gpu/drm/i915/i915_sysfs.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   4196	    928	      0	   5124	   1404	drivers/gpu/drm/i915/i915_sysfs.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/13b5c875e677c10e6257be4fac31b2b6c77a494f.1499079914.git.arvind.yadav.cs@gmail.com