anv: Account for the header in anv_state_stream_alloc

anv: Account for the header in anv_state_stream_alloc

If we have an allocation that's exactly the block size, we end up
computing a new block size to allocate that's exactly the block size,
add in the header, and then assert fail.  When computing the block size,
we need to account for the header.

Fixes: 955127db937 "anv/allocator: Add support for large stream..."
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4336>