drm/i915: Fix kerneldoc for i915_gem_shrink_all

I've botched this in

commit eb0b44ad
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Mar 18 14:47:59 2015 +0100

    drm/i915: kerneldoc for i915_gem_shrinker.c

so let's fix it.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>