drm/sun4i: constify drm_plane_helper_funcs

drm_plane_helper_funcs are not supposed to change at runtime.
All functions working with drm_plane_helper_funcs provided by
<drm/drm_plane_helper.h> work with const drm_plane_helper_funcs.
So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
    981	     40	      0	   1021	    3fd drivers/gpu/drm/sun4i/sun4i_layer.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   1021	      0	      0	   1021	    3fd drivers/gpu/drm/sun4i/sun4i_layer.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>