drm/exynos: use sgt instead of pages for framebuffer address

The 'pages' structure in the exynos gem buffer has been
removed. So we get the fix.smem_start from the first sgl
of the scatter gather table.

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>