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