drm/i915/selftests/blt: add some kthreads into the mix

We can be more aggressive in our testing by launching a number of
kthreads, where each is submitting its own copy or fill batches on a set
of random sized objects. Also since the underlying fill and copy batches
can be pre-empted mid-batch(for particularly large objects), throw in a
random mixture of ctx priorities per thread to make pre-emption a
possibility.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191025172511.25742-1-matthew.auld@intel.com