drm/i915/selftests: Refactor sibling selection

Tvrtko spotted that some selftests were using 'break' not 'continue',
which will fail for discontiguous engine layouts such as on Icelake
(which may have vcs0 and vcs2).

Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200518102911.3463-1-chris@chris-wilson.co.uk