i915/gem_ctx_switch: Use minimum qlen over all engines and measure switches

Not all engines are created equal, and our weighting ends up favouring
the many faster xCS rings at the expense of RCS. Our qlen estimation
also failed to factor in the context switch overhead, which is a
significant factor for nop batches. So we oversubscribe the number of
batches submitted to RCS and end up waiting for those to complete at the
end of our subtest timeslice.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Caz Yokoyama <caz.yokoyama@intel.com>
Reviewed-by: Caz Yokoyama <caz.yokoyama@intel.com>

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