fix build error:
drivers/gpu/drm/rockchip/rockchip_drm_fb.c:
In function 'rockchip_drm_fb_destroy':
drivers/gpu/drm/rockchip/rockchip_drm_fb.c:74:21:
error: 'struct rockchip_drm_fb' has no member named 'sgt'
Change-Id: I413391291be21ebf452bde340a788dcf2d1c76d4 Signed-off-by: Mark Yao <mark.yao@rock-chips.com>