drm/ast: use DRM_GEM_SHMEM_DRIVER_OPS_NO_MAP_SGT

Import dmabuf without mapping its sg_table to avoid issues likes:
  ast 0000:07:00.0: swiotlb buffer is full (sz: 3145728 bytes), total 32768 (slots), used 0 (slots)

Signed-off-by: Shixiong Ou <oushixiong@kylinos.cn>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20250522070714.439824-2-oushixiong1025@163.com