drm/cma: remove to make sg_table when gem cma is created

The sg_table made when gem cma is created isn't used anywhere. The sgt
of struct drm_gem_cma_object will have only sg_tabel imported.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>