drm/exynos: fix ref count leak in mic_pre_enable

in mic_pre_enable, pm_runtime_get_sync is called which
increments the counter even in case of failure, leading to incorrect
ref count. In case of failure, decrement the ref count before returning.

Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>