radv/winsys: fix resetting the number of padded IB words

The number of padded words is per IB, so it should be reset.

Found with Valgrind and dEQP-VK.api.command_buffers.record_many_draws*

Cc: 21.1 mesa-stable
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <!10376>
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