drm/i915: don't call intel_fbc_update() at intel_unpin_work_fn()

Because intel_unpin_work_fn() already calls
intel_frontbuffer_flip_complete() which will call intel_fbc_flush()
which will call intel_fbc_update() when needed.

We couldn't fix this previously due to the fact that FBC was not
properly behaving as intended on frontbuffer flushes, but now that
this is fixed, we can remove the additional call.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>