drm/exynos: Use devm_kzalloc in exynos_drm_hdmi.c file

devm_kzalloc is a device managed function and makes freeing and error
handling simpler.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>