swr: Fix build with llvm-9.0 again.

Commit 6f7306c0 ("swr/rast: Refactor memory API between rasterizer
core and swr") unintentionally removed changes for llvm-9.0.

Fixes: 6f7306c0 ("swr/rast: Refactor memory API between rasterizer core and swr")
Fixes: 5dd9ad15 ("swr/rasterizer: Better implementation of scatter")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Jan Zielinski <jan.zielinski@intel.com>
(cherry picked from commit 3664a660)