drm/panfrost: Remove set but not used variable 'bo'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/panfrost/panfrost_job.c: In function 'panfrost_job_cleanup':
drivers/gpu/drm/panfrost/panfrost_job.c:278:31: warning:
 variable 'bo' set but not used [-Wunused-but-set-variable]

commit bdefca2d

 ("drm/panfrost: Add the panfrost_gem_mapping concept")
involved this unused variable.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Alyssas Rosenzweig <alyssa.rosenzweig@collabora.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200203152724.42611-1-yuehaibing@huawei.com