drm/exynos/ipp: add file checks for ioctls

Process should not have access to ipp nodes created by another
process. The patch adds necessary checks.
It also simplifies lookup for command node.

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>