drm/i915: limit the async bind to bind_async_flags

If the vm doesn't request async binding, like for example with the dpt,
then we should be able to skip the async path and avoid calling
i915_vm_lock_objects() altogether. Currently if we have a moving fence
set for the BO(even though it might have signalled), we still take the
async patch regardless of the bind_async setting, and then later still
end up just doing i915_gem_object_wait_moving_fence() anyway.

Alternatively we would need to add dummy scratch object which can be
locked, just for the dpt.

Suggested-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220304095934.925036-2-matthew.auld@intel.com