tests/perf: respect gen11 restriction in disable subslices

In particular don't try to enable more than half the subslices.

v2: Pull the Gen11 out of subprocess to display selected configuration
    Use DIV_ROUND_UP() for Gen11 with odd number of subslices

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/1543
Fixes: 779d43cd ("tests/perf: add global sseu parameter tests")
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v1)