drm/i915/selftests: Select a random engine for testing memory regions

Use any blitter engine at random for prefilling the memory region.

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/20191027225808.19437-5-chris@chris-wilson.co.uk