drm/exynos/ipp: fix get_property IOCTL

Due to incorrect assignment in EXYNOS_IPP_GET_PROPERTY
IOCTL handler this IOCTL did not work at all.
The patch fixes it.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>