anv: use helper function to get the buffer size

This ensures we get a properly aligned size for the buffer so we don't
trip over HW limits for push constants.

Closes #3703
Fixes dEQP-VK.robustness.image_robustness.push.* on HSW

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <!9699>
35 jobs for !9699 with hsw-robustness in 8 minutes and 43 seconds (queued for 3 seconds)
latest detached
Status Job ID Name Coverage
  Sanity
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sanity

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  Container
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aarch64
arm_build

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arm_test-base

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windows shell 1809 mesa
windows_build_vs2019

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x86_build-base

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x86_test-base

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android_build

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arm64_test

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armhf_test

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i386_build

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kernel+rootfs_amd64

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aarch64
kernel+rootfs_arm64

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aarch64
kernel+rootfs_armhf

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ppc64el_build

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s390x_build

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x86_build

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x86_test-gl

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x86_test-vk

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meson-clang

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meson-clover

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meson-clover-testing

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meson-gallium

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meson-release

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meson-testing

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meson-testing-asan

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meson-vulkan

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  Build Misc
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meson-android

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aarch64
meson-arm64

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aarch64
meson-arm64-asan

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aarch64
meson-arm64-build-test

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aarch64
meson-armhf

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meson-i386

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meson-mingw32-x86_64

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kvm
meson-ppc64el

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kvm
meson-s390x

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