drm: Free the writeback_job when it with an empty fb

Adds the check if the writeback_job with an empty fb, then it should
be freed in atomic_check phase.

With this change, the driver users will not check empty fb case any more.
So refined accordingly.

Signed-off-by: Lowry Li (Arm Technology China) <lowry.li@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@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/1564571048-15029-2-git-send-email-lowry.li@arm.com