drm/exynos/hdmi: remove private lock code

Most of the code is called by drm core framework, so it is already synchronized.
The only async function is irq routine which only calls drm framework so it
does not need to be synchronized.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>