drm/i915: Make intel_unpin_fb_obj() static

This function is not used outside of intel_display.c since;

  commit cf4c7c12258ed9367f4fc45238f5f50d2db892c1
  Author: Matt Roper <matthew.d.roper@intel.com>
  Date:   Thu Dec 4 10:27:42 2014 -0800

      drm/i915: Make all plane disables use 'update_plane' (v5)

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index d08f8ee..3b0fe9f 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -2287,7 +2287,7 @@
 	return ret;
 }
 
-void intel_unpin_fb_obj(struct drm_i915_gem_object *obj)
+static void intel_unpin_fb_obj(struct drm_i915_gem_object *obj)
 {
 	WARN_ON(!mutex_is_locked(&obj->base.dev->struct_mutex));