pan/bi: Add some zero bytes after shaders on Bifrost

Bifrost will prefetch bytes after the end of shaders, so make sure
these bytes are allocated and zeroed.

Fixes GPU faults in Xonotic.

Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <!8428>
42 jobs for !8428 with shaderbo in 16 minutes and 44 seconds (queued for 5 seconds)
merge request