drm/i915: Flush the PTEs after updating them before suspend

drm/i915: Flush the PTEs after updating them before suspend

As we use WC updates of the PTE, we are responsible for notifying the
hardware when to flush its TLBs. Do so after we zap all the PTEs before
suspend (and the BIOS tries to read our GTT).

Fixes a regression from

commit 828c79087cec61eaf4c76bb32c222fbe35ac3930
Author: Ben Widawsky <[email protected]>
Date:   Wed Oct 16 09:21:30 2013 -0700

    drm/i915: Disable GGTT PTEs on GEN6+ suspend

that survived and continue to cause harm even after

commit e568af1c626031925465a5caaab7cca1303d55c7
Author: Daniel Vetter <[email protected]>
Date:   Wed Mar 26 20:08:20 2014 +0100

    drm/i915: Undo gtt scratch pte unmapping again

v2: Trivial rebase.
v3: Fixes requires pointer dances.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=82340
Tested-by: [email protected]
Signed-off-by: Chris Wilson <[email protected]>
Cc: [email protected]
Cc: Takashi Iwai <[email protected]>
Cc: Paulo Zanoni <[email protected]>
Cc: Todd Previte <[email protected]>
Cc: Daniel Vetter <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
Signed-off-by: Jani Nikula <[email protected]>