i965: Call prepare_external after implicit window-system MSAA resolves

This fixes some rendering corruption in a couple of Android apps that
use window-system MSAA.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104741


Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
(cherry picked from commit 2f7205be)