drm/exynos: g2d: fix memory leak to userptr

This patch releases a vma object when cleaning up userptr resources.
A new vma object was allocated and copied when getting userptr pages
so the new vma object should be freed properly if the userptr pages
aren't used anymore.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>