drm: Clear the fence pointer when writeback job signaled

drm: Clear the fence pointer when writeback job signaled

BugLink: https://bugs.launchpad.net/bugs/1850456
[ Upstream commit b1066a123538044117f0a78ba8c6a50cf5a04c86 ]

During it signals the completion of a writeback job, after releasing
the out_fence, we'd clear the pointer.

Check if fence left over in drm_writeback_cleanup_job(), release it.

Signed-off-by: Lowry Li (Arm Technology China) <lowry.li@arm.com>
Reviewed-by: Brian Starkey <brian.starkey@arm.com>
Reviewed-by: James Qian Wang (Arm Technology China) <james.qian.wang@arm.com>
Signed-off-by: james qian wang (Arm Technology China) <james.qian.wang@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1564571048-15029-3-git-send-email-lowry.li@arm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>