drm/panfrost: Fix runtime PM imbalance on error

The caller expects panfrost_job_hw_submit() to increase
runtime PM usage counter. The refcount decrement on the
error branch of WARN_ON() will break the counter balance
and needs to be removed.

Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200522134109.27204-1-dinghao.liu@zju.edu.cn