drm/ttm: Fix a regression causing kernel oops'es

The branch is explicitly taken if ttm == NULL which means that to avoid
a null pointer reference the ttm object can not be used inside. Switch
back to dst_mem to avoid kernel oops'es.

This fixes kernel oops'es with any buffer objects which don't have ttm_tt,
e.g. with vram based screen objects on vmwgfx.
Signed-off-by: Zack Rusin <zackr@vmware.com>
Fixes: e3c92eb4 ("drm/ttm: rework on ttm_resource to use size_t type")
Cc: Somalapuram Amaranath <Amaranath.Somalapuram@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230111175015.1134923-1-zack@kde.orgSigned-off-by: Christian König <christian.koenig@amd.com>

drivers/gpu/drm/ttm/ttm_bo_util.c

@@ -173,7 +173,7 @@ int ttm_bo_move_memcpy(struct ttm_buffer_object *bo,
 
 	clear = src_iter->ops->maps_tt && (!ttm || !ttm_tt_is_populated(ttm));
 	if (!(clear && ttm && !(ttm->page_flags & TTM_TT_FLAG_ZERO_ALLOC)))
-		ttm_move_memcpy(clear, ttm->num_pages, dst_iter, src_iter);
+		ttm_move_memcpy(clear, PFN_UP(dst_mem->size), dst_iter, src_iter);
 
 	if (!src_iter->ops->maps_tt)
 		ttm_kmap_iter_linear_io_fini(&_src_iter.io, bdev, src_mem);