drm: renesas: rz-du: Support dmabuf import

The rz-du driver uses GEM DMA helpers, but does not implement the
drm_driver .gem_prime_import_sg_table operation. This  prevents
importing dmabufs. Fix it by implementing the missing operation using
the DRM_GEM_DMA_DRIVER_OPS_WITH_DUMB_CREATE() helper macro.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Tested-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com> # RZ/V2H + DSI
Tested-by: Tommaso Merciai <tommaso.merciai.xr@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250321104615.31809-1-laurent.pinchart+renesas@ideasonboard.com