drm/pl111: make structure mode_config_funcs static

structure mode_config_funcs can be made static as it does not need to be
in global scope.  Fixes sparse warning:

warning: symbol 'mode_config_funcs' was not declared. Should it be static?
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170519111018.19641-1-colin.king@canonical.com

drivers/gpu/drm/pl111/pl111_drv.c

@@ -72,7 +72,7 @@
 
 #define DRIVER_DESC      "DRM module for PL111"
 
-struct drm_mode_config_funcs mode_config_funcs = {
+static struct drm_mode_config_funcs mode_config_funcs = {
 	.fb_create = drm_fb_cma_create,
 	.atomic_check = drm_atomic_helper_check,
 	.atomic_commit = drm_atomic_helper_commit,