drm/atmel-hlcdc : constify drm_plane_helper_funcs and drm_plane_funcs.

drm_plane_helper_funcs and drm_plane_funcsare not supposed to change
at runtime. All functions working with drm_plane_helper_funcs and
drm_plane_funcs work with const. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
   6072	    596	      0	   6668	   1a0c atmel_hlcdc_plane.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   6218	    436	      0	   6654	   19fe atmel_hlcdc_plane.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/646415a3b2e62182f85254115e8491e5caf4b2c7.1499098826.git.arvind.yadav.cs@gmail.com