pvr: Correct calculations in pvr_unwind_rects()

Matt Coster requested to merge MTCoster/mesa:dev/unwind-rects-fix into main

What does this MR do and why?

Fixes 6d3d1f88 (pvr: Fix rect splitting logic in pvr_unwind_rects()) and cleans up the logic in that function.

Signed-off-by: Matt Coster <matt.coster@imgtec.com>
Reported-by: Soroush Kashani <soroush.kashani@imgtec.com>
Reviewed-by: Karmjit Mahil <Karmjit.Mahil@imgtec.com>
Edited by Matt Coster

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