drm/exynos: replace request_threaded_irq with devm function

devm_request_threaded_irq is used instead of request_threaded_irq
and free_irq is removed.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>