film_grain: don't attempt copying chroma planes for I400

Fixes #191
12 jobs for master in 10 minutes and 53 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Build
passed build-debian #236903
debian amd64

00:00:33

passed build-debian-aarch64 #236907
aarch64 debian

00:01:26

passed build-debian-aarch64-clang-5 #236908
aarch64 debian

00:00:59

passed build-debian-static #236904
debian amd64

00:00:33

passed build-debian-werror #236910
aarch64 debian

00:00:32

passed build-macos #236909
macos

00:00:25

passed build-win32 #236905
win32

00:00:30

passed build-win64 #236906
win64

00:00:33

 
  Test
passed test-debian #236911
debian amd64

00:00:51

passed test-debian-asan #236912
debian amd64

00:02:19

passed test-debian-msan #236913
debian amd64

00:01:07

passed test-debian-ubsan #236914
debian amd64

00:01:25