st/mesa: clean up atomic vs ssbo code

st/mesa: clean up atomic vs ssbo code

This makes the code a bit easier to follow; we first set up
MaxShaderStorageBlocks, then we either set up a dedicated
MaxAtomicBuffers, or we split MaxShaderStorageBlocks in two.

While we're at it, also make the SSBO-splitting code tolerate the
hypothetical case of having an odd number of SSBOs without incorrectly
dropping the last SSBO.

This has the nice result that the SSBOs and atomic buffers are dealt
with almost completely orthogonally, easing some upcoming patches.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>