drm/i915: make skl_ddb_add_affected_planes static

Fix sparse warning:

drivers/gpu/drm/i915/intel_pm.c:3970:1: warning: symbol
'skl_ddb_add_affected_planes' was not declared. Should it be static?

Fixes: 7f60e200e254 ("drm/i915/gen9: only add the planes actually affected by ddb changes")
Cc: Lyude <cpaul@redhat.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1475573357-30562-1-git-send-email-jani.nikula@intel.com

drivers/gpu/drm/i915/intel_pm.c

@@ -3966,7 +3966,7 @@ pipes_modified(struct drm_atomic_state *state)
 	return ret;
 }
 
-int
+static int
 skl_ddb_add_affected_planes(struct intel_crtc_state *cstate)
 {
 	struct drm_atomic_state *state = cstate->base.state;