drm/exynos: remove call to drm_gem_free_mmap_offset()

The drm_gem_object_release() function already performs this cleanup,
so there is no reason to do it explicitly.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>