freedreno/crashdec: Use register for RB rptr

The register is usually a few dwords ahead of the memory value used by
the kernel, which can lead to an inaccurate calculation of where the SQE
is.

To compensate for the more accurate rptr, increase the lookback
slightly.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34063>
4 files changed