drm/vmwgfx: Make the driver work without the dummy resources

In commit 180253782038 ("drm/ttm: stop allocating dummy resources during BO creation")
ttm stopped allocating dummy resources but vmwgfx was never ported to
handle it. Make the driver treat null resources as initial creation and
port code to handle null resources in general.

Fixes kernel oops'es on boot with vmwgfx.

Signed-off-by: Zack Rusin <[e-mail]>
Fixes: 180253782038 ("drm/ttm: stop allocating dummy resources during BO creation")
Cc: Christian König <[e-mail]>
Cc: Matthew Auld <[e-mail]>
Cc: Nirmoy Das <[e-mail]>
Cc: Christian Koenig <[e-mail]>
Cc: Huang Rui <[e-mail]>
Cc: [e-mail]
Reviewed-by: Christian König <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/vmwgfx/vmwgfx_resource.c

···
 	mutex_lock(&dev_priv->binding_mutex);
 
 	/* If BO is being moved from MOB to system memory */
-	if (new_mem->mem_type == TTM_PL_SYSTEM &&
+	if (old_mem &&
+	    new_mem->mem_type == TTM_PL_SYSTEM &&
 	    old_mem->mem_type == VMW_PL_MOB) {
 		struct vmw_fence_obj *fence;
 

drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c

···
 		    struct ttm_resource *new_mem,
 		    struct ttm_place *hop)
 {
-	struct ttm_resource_manager *old_man = ttm_manager_type(bo->bdev, bo->resource->mem_type);
-	struct ttm_resource_manager *new_man = ttm_manager_type(bo->bdev, new_mem->mem_type);
-	int ret;
+	struct ttm_resource_manager *new_man;
+	struct ttm_resource_manager *old_man = NULL;
+	int ret = 0;
+
+	new_man = ttm_manager_type(bo->bdev, new_mem->mem_type);
+	if (bo->resource)
+		old_man = ttm_manager_type(bo->bdev, bo->resource->mem_type);
 
 	if (new_man->use_tt && !vmw_memtype_is_system(new_mem->mem_type)) {
 		ret = vmw_ttm_bind(bo->bdev, bo->ttm, new_mem);
···
 			return ret;
 	}
 
+	if (!bo->resource || (bo->resource->mem_type == TTM_PL_SYSTEM &&
+			      bo->ttm == NULL)) {
+		ttm_bo_move_null(bo, new_mem);
+		return 0;
+	}
+
 	vmw_move_notify(bo, bo->resource, new_mem);
 
-	if (old_man->use_tt && new_man->use_tt) {
+	if (old_man && old_man->use_tt && new_man->use_tt) {
 		if (vmw_memtype_is_system(bo->resource->mem_type)) {
 			ttm_bo_move_null(bo, new_mem);
 			return 0;