arm64: filmgrain: Share the prologue of the fgy function

This is the same as what was done for the fguv function, to reduce
the amount of space used for it (and also simplifying the calling
code).

This gives no significant slowdown for the case currently benchmarked
by checkasm, while shrinking the code produced by film_grain.S by
320 bytes.
38 jobs for arm64-fguv in 6 minutes and 5 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Style
passed #615722
amd64 docker
style-check

00:00:07

passed #615723
amd64 docker
x86inc-check

00:00:07

 
  Build
passed #615738
amd64 docker
build-android-aarch64

00:00:10

passed #615737
amd64 docker
build-android-armv7

00:00:11

passed #615724
amd64 avx2 docker
build-debian

00:00:24

passed #615739
aarch64 docker
build-debian-aarch64

00:00:16

passed #615740
aarch64 docker
build-debian-aarch64-clang-5

00:00:14

passed #615743
armv7 docker
build-debian-armv7

00:01:51

passed #615744
armv7 docker
build-debian-armv7-clang-5

00:00:25

passed #615731
amd64 docker
build-debian-bitdepth-16

00:00:10

passed #615730
amd64 docker
build-debian-bitdepth-8

00:00:21

passed #615728
amd64 docker
build-debian-examples

00:00:17

passed #615726
amd64 avx2 docker
build-debian-illegal-instructions

00:01:35

passed #615729
amd64 docker
build-debian-no-tools

00:00:20

passed #615746
ppc64le docker
build-debian-ppc64le

00:01:18

passed #615725
amd64 docker
build-debian-static

00:00:23

passed #615742
aarch64 docker
build-debian-werror

00:00:09

passed #615727
amd64 docker
build-debian32

00:00:26

passed #615741
catalina amd64
build-macos

00:00:32

passed #615747
amd64 docker
build-pages

00:00:09

passed #615745
amd64 docker
build-ubuntu-snap

00:00:18

passed #615735
amd64 docker
build-win-arm32

00:00:10

passed #615736
amd64 docker
build-win-arm64

00:00:17

passed #615732
amd64 docker
build-win32

00:00:35

passed #615733
amd64 docker
build-win32-unaligned-stack

00:00:30

passed #615734
amd64 docker
build-win64

00:00:40

 
  Test
passed #615748
amd64 docker
test-debian

00:01:14

95.8417%
passed #615757
aarch64 docker
test-debian-aarch64

00:00:26

passed #615759
armv7 docker
test-debian-armv7-clang-5

00:04:05

passed #615752
amd64 docker
test-debian-asan

00:02:02

passed #615749
avx2 amd64 docker
test-debian-asm

00:01:08

passed #615753
amd64 docker
test-debian-msan

00:00:58

passed #615758
ppc64le docker
test-debian-ppc64le

00:02:48

passed #615755
amd64 docker
test-debian-tsan

00:03:22

passed #615754
amd64 docker
test-debian-ubsan

00:01:29

passed #615751
amd64 avx2 docker
test-debian-unaligned-stack

00:00:58

passed #615750
avx2 amd64 docker
test-debian32-asm

00:01:09

passed #615756
amd64 avx2 docker
test-win64

00:03:27