drm/i915: Protect WC stash allocation against direct reclaim

As we attempt to allocate pages for use in a new WC stash, direct
reclaim may run underneath us and fill up the WC stash. We have to be
careful then not to overflow the pvec.

Fixes: 66df1014

 ("drm/i915: Keep a small stash of preallocated WC pages")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103109
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180121173143.17090-1-chris@chris-wilson.co.uk