i965: Use PTE MOCS for all external buffers

i965: Use PTE MOCS for all external buffers

We were already using PTE for all render targets in case one happened to
get scanned out.  However, this still wasn't 100% correct because there
are still possibly cases where we may want to texture from an external
buffer even though we don't know the caching mode.  This can happen, for
instance, on buffers imported from another GPU via prime.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101691
Cc: "17.3" <mesa-stable@lists.freedesktop.org>
Tested-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>