drm/i915/vma: document struct i915_vma_resource wakeref member

drivers/gpu/drm/i915/i915_vma_resource.h:129: warning: Function parameter or member 'wakeref' not described in 'i915_vma_resource'
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/50389365e095dc564ab5f1f1e3647934163ffefa.1683041799.git.jani.nikula@intel.com

drivers/gpu/drm/i915/i915_vma_resource.h

@@ -47,6 +47,7 @@ struct i915_page_sizes {
  * @chain: Pointer to struct i915_sw_fence used to await dependencies.
  * @rb: Rb node for the vm's pending unbind interval tree.
  * @__subtree_last: Interval tree private member.
+ * @wakeref: wakeref.
  * @vm: non-refcounted pointer to the vm. This is for internal use only and
  * this member is cleared after vm_resource unbind.
  * @mr: The memory region of the object pointed to by the vma.