drm/i915/selftests: Reorder request allocation vs vma pinning

Impose a restraint that we have all vma pinned for a request prior to
its allocation. This is to simplify request construction, and should
facilitate unravelling the lock interdependencies later.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181204141522.13640-3-chris@chris-wilson.co.uk