drm/i915: Fix docbook descriptions for i915_gem_shrinker

Fixes the following htmldocs warning:
drivers/gpu/drm/i915/gem/i915_gem_shrinker.c:102: warning: Function parameter or member 'ww' not described in 'i915_gem_shrink'

Fixes: cf41a8f1

 ("drm/i915: Finally remove obj->mm.lock.")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210421120938.546076-1-maarten.lankhorst@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
(cherry picked from commit 772f7bb7

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>