drm: armada: Constify drm_prop_enum_list structures.

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

File size before:
   text	   data	    bss	    dec	    hex	filename
   3594	    176	      0	   3770	    eba drivers/gpu/drm/armada/armada_overlay.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   3722	     48	      0	   3770	    eba drivers/gpu/drm/armada/armada_overlay.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/dbeb176765bda8eaa9efdaa2dcd14c7bbae39cfa.1498905467.git.arvind.yadav.cs@gmail.com