drm/i915: Make intel_unpin_fb_obj() static

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

  commit cf4c7c12
  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>