drm/i915/selftests: Reduce oversaturation of request smoketesting

The goal in launching the request smoketest is to have sufficient tasks
running across the system such that we are likely to detect concurrency
issues. We aim to have 2 tasks using the same engine, gt, device (each
level of locking around submission and signaling) running at the same
time. While tasks may not be running all the time as they synchronise
with the gpu, they will be running most of the time, in which case
having many more tasks than cores available is wasteful (and
dramatically increases the workload causing excess runtime). Aim to
limit the number of tasks such that there is at least 2 running per
engine, spreading surplus cores around the engines (rather than running
a task per core per engine.)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Tested-by: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221102155709.31717-1-nirmoy.das@intel.com