drm/i915: Ironlake do_idle_maps w/a may be called w/o struct_mutex

drm/i915: Ironlake do_idle_maps w/a may be called w/o struct_mutex

Since commit 3bc50ee265e2 ("drm/i915: Move object backing storage
manipulation to its own locking"), i915_gem_object_put_pages() and
specifically the i915_gem_gtt_finish_pages() may be called from outside
of the struct_mutex and so we can no longer pass I915_WAIT_LOCKED to
i915_gem_wait_for_idle.

Fixes: 3bc50ee265e2 ("drm/i915: Move object backing storage manipulation to its own locking")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v4.10+
Link: http://patchwork.freedesktop.org/patch/msgid/20170330085341.20311-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>