drm/exynos: g2d: fix missing unlock on error in g2d_runqueue_worker()

Add the missing unlock before return from function g2d_runqueue_worker()
in the error handling case.

Fixes: 445d3bed ("drm/exynos: use pm_runtime_resume_and_get()")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>