drm/i915: make structure intel_sprite_plane_funcs static

The structure intel_sprite_plane_funcs is local to the source
and does not need to be in global scope, so make it static.

Cleans up sparse warning:
symbol 'intel_sprite_plane_funcs' was not declared. Should it be static?
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170811134938.4183-1-colin.king@canonical.com

drivers/gpu/drm/i915/intel_sprite.c

@@ -1184,7 +1184,7 @@ static bool intel_sprite_plane_format_mod_supported(struct drm_plane *plane,
 	unreachable();
 }
 
-const struct drm_plane_funcs intel_sprite_plane_funcs = {
+static const struct drm_plane_funcs intel_sprite_plane_funcs = {
         .update_plane = drm_atomic_helper_update_plane,
         .disable_plane = drm_atomic_helper_disable_plane,
         .destroy = intel_plane_destroy,