drm/exynos/ipp: move file reference from memory to command node

Command node should contain file reference to distinguish commands
created by different processes.

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>