evictions: Limit the number of minor eviction surfaces to fit in RAM

evictions: Limit the number of minor eviction surfaces to fit in RAM

We allocate more surfaces than used in a single pass in order to stress
the eviction code between batches. The intent here is not to exercise
swapping, and we fail to check that there is enough swap+memory to hold
all our surfaces. So limit the number of surfaces we allocate to fit
into RAM, and then require that the number of surfaces we need for
testing is less than the number of surfaces we can allocate.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>