[HDRP] Fix custom pass buffers not bound after creation

[alelievr]

Purpose of this PR

Fixed custom pass custom buffer not bound after being created inside a custom pass.

https://unity.slack.com/archives/C6Y79CZM0/p1628781340168900

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Testing status

Tested the repro case on slack

https://yamato.cds.internal.unity3d.com/jobs/902-Graphics/tree/hd%252Ffix%252Fcustom-pass-buffers-not-bound-after-creation/.yamato%252Fall-hdrp.yml%2523PR_HDRP_trunk/8729485/job

[github-actions]

Hi! This comment will help you figure out which jobs to run before merging your PR. The suggestions are dynamic based on what files you have changed.
Link to Yamato: https://yamato.cds.internal.unity3d.com/jobs/902-Graphics
Search for your PR branch using the sidebar on the left, then add the following segment(s) to the end of the URL (you may need multiple tabs depending on how many packages you change)

HDRP
/.yamato%252Fall-hdrp.yml%2523PR_HDRP_trunk
With changes to HDRP packages, you should also run
/.yamato%252Fall-lightmapper.yml%2523PR_LightMapper_trunk

Depending on the scope of your PR, you may need to run more jobs than what has been suggested. Please speak to your lead or a Graphics SDET (#devs-graphics-automation) if you are unsure.