drm/i915/selftests: Use a random engine for GEM coherency tests

Select a random user accessible engine for checking coherency results.
While we should check all engines, we use a random selection so that
over repeated runs we cover all.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191027225808.19437-4-chris@chris-wilson.co.uk