anv/meta: Create 8 pipelines for color clears

anv/meta: Create 8 pipelines for color clears

This prepares for moving the clear ops from the start of the render pass
into each subpass.

Pipeline N will be used to clear color attachment N of the current
subpass. Currently meta color clears still create a throwaway subpass
with exactly one attachment, so currently only pipeline 0 is used.

This is an ugly hack to workaround the compiler's current inability to
dynamically set the render target index in the render target write
message.