Fix for vulkan sysgpu backend when only some attachments are cleared

[platypro]

Fix for issue I ran into when I tried to "load" a color attachment, but "clear" the depth attachment. I got a validation error saying that all buffers must be represented in p_clear_values/clear_value_count so the driver can just jump to find the matching attachment.

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[emidoots]

Thanks for sending this, I need to check if this alters the behavior of any examples we have - not sure the underlying intention of this logic before