drm/i915: fix intel_frontbuffer_flip documentation

Reported by the kbuild test robot.

Regression introduced by:
commit fdbff928


Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Jun 18 11:23:24 2015 +0200
    drm/i915: Clear fb_tracking.busy_bits also for synchronous flips

(I reviewed this commit, so it's also my fault)

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