igt/gem_concurrent_blit: Limit number of reads for dmabufs

vgem provides WC mmap on its dmabufs, so reading and comparing these is
slow. We can employ a similar w/a to the other WC mmaps (i.e. mmap__gtt
or mmap__wc) and only check one pixel per page. This will speed up those
tests, but I don't know if it makes detecting coherency issues more or
less likely. :|

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

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