perf test amd: Skip amd-ibs-period test on kernel < v6.15

Bunch of IBS kernel fixes went in v6.15-rc1 [1].

The amd-ibs-period test will fail without those kernel patches.

Skip the test on system running kernel older than v6.15 to distinguish
genuine new failures vs known failure due to old kernel.

Since all the related IBS fixes went in -rc1 itself, the ">= 6.15" check
will work for any custom compiled v6.15-* kernel as well.

Reported-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Suggested-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
Closes: https://lore.kernel.org/r/aCfuGXUnNIbnYo_r@x1
Link: https://lore.kernel.org/r/20250115054438.1021-1-ravi.bangoria@amd.com

 [1]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>