intel/perf: fix regex escaping

intel/perf: fix regex escaping

`\$` is interpreted before being passed to `re.search()`, but luckily
for us the escape is also invalid and because of that, python 3.12+
warns us about it.

Use a raw string instead, so that the `\` is passed untouched to
`re.search()`.

Fixes: aa04b47c6e43e6cc3561 ("intel/perf: add support for GtSlice/GtSliceXDualsubsliceY variables")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26355>
(cherry picked from commit 19420731123e850e10c68eb3cd672b00561c48d7)