drm/i915/fbc: make sure we cancel the work function at fbc_disable

Just to be sure nothing will survive a module unload. We need to do
this after the unlock in order to make sure the function won't get
stuck trying to grab the lock we already own while we wait for it to
finish.

Reported-by: Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453210558-7875-15-git-send-email-paulo.r.zanoni@intel.com