drm/i915/selftests: Exercise parallel blit operations on a single ctx

Make sure that our code is robust enough to handle multiple threads
trying to clear objects for a single client context. This brings the joy
of a shared GGTT to all!

References: https://bugs.freedesktop.org/show_bug.cgi?id=112176


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191111122706.28292-1-chris@chris-wilson.co.uk