freedreno/gmem: rework gmem layout algo

And try a bit harder to find an optimal layout.  Improves on a sub-
optimal layout we arrive at in the 4 MRT pass in manhattan, picking
up a bit more than 3%.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4976>
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