drm/i915: Only activate i915_active debugobject once

The point of debug_object_activate is to mark the first, and only the
first, acquisition. The object then remains active until the last
release.  However, we marked up all successful first acquires even though
we allowed concurrent parties to try and acquire the i915_active
simultaneously (serialised by the i915_active.mutex).

Testcase: igt/gem_mmap_gtt/fault-concurrent
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190827132631.18627-1-chris@chris-wilson.co.uk