UPSTREAM: drm/fence: release fence reference when canceling event

If the event gets canceled we also need to put away the fence
reference it holds.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1476975005-30441-3-git-send-email-gustavo@padovan.org


(cherry picked from commit 838de39fc9516470b1410cc068c7f41daec69265)
Signed-off-by: Marissa Wall <marissaw@google.com>

Change-Id: I421435486b4e40e342564fa5c4327ff3ab62498c