drm/exynos: remove unnecessary sg_alloc_table call

The function dma_get_sgtable will allocate a sg table internally so
it is not necessary to allocate a sg table before it. The unnecessary
'sg_alloc_table' call is removed.

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