drm/i915/fbdev: Call intel_unpin_fb_obj() on release

When releasing the intel_fbdev, we should unpin the framebuffer that we
pinned during construction.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1461833819-3991-1-git-send-email-chris@chris-wilson.co.uk