malidp: Fix NULL vs IS_ERR() checking

The get_sg_table() function does not return NULL.
It returns error pointers.
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://lore.kernel.org/dri-devel/20211213072115.18098-1-linmq006@gmail.com/

drivers/gpu/drm/arm/malidp_planes.c

@@ -344,7 +344,7 @@ static bool malidp_check_pages_threshold(struct malidp_plane_state *ms,
 		else
 			sgt = obj->funcs->get_sg_table(obj);
 
-		if (!sgt)
+		if (IS_ERR(sgt))
 			return false;
 
 		sgl = sgt->sgl;