drm/i915/gtt: Make sure the gen6 ppgtt is bound before first use

As we remove the struct_mutex protection from around the vma pinning,
counters need to be atomic and aware that there may be multiple threads
simultaneously active.

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/20190913064200.24297-1-chris@chris-wilson.co.uk