drm/exynos: use release_mem_region instead of release_resource

To make a api pair of request_mem_region and release_mem_region,
release_mem_region is used instead of release_resource.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>