drm/exynos: Fix error routine to getting dma addr.

This patch fixes error routine when g2d_userptr_get_dma_add is failed.
When sg_alloc_table_from_pages() is failed, it doesn't call
sg_free_table() anymore.

Signed-off-by: YoungJun Cho <yj44.cho@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>