drm: radeon: 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
  18276	    384	      0	  18660	   48e4	drivers/gpu/drm/radeon/radeon_display.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
  18660	      0	      0	  18660	   48e4	drivers/gpu/drm/radeon/radeon_display.o

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>