drm/exynos: free sg object if dma_map_sg is failed

This patch releases sgt's sg object allocated by sgt_alloc_table
correctly.

When exynos_gem_map_dma_buf was called by dma_buf_map_attachmemt(),
the sgt's sg object was allocated by sg_alloc_tale() so
if dma_map_sg() is failed, the sg object should be released.

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