agp/intel: Flush all chipset writes after updating the GGTT

Before accessing the GGTT we must flush the PTE writes and make them
visible to the chipset, or else the indirect access may end up in the
wrong page. In commit 3497971a ("agp/intel: Flush chipset writes
after updating a single PTE"), we noticed corruption of the uploads for
pwrite and for capturing GPU error states, but it was presumed that the
explicit calls to intel_gtt_chipset_flush() were sufficient for the
execbuffer path. However, we have not been flushing the chipset between
the PTE writes and access via the GTT itself.

For simplicity, do the flush after any PTE update rather than try and
batch the flushes on a just-in-time basis.

References: 3497971a ("agp/intel: Flush chipset writes after updating a single PTE")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: drm-intel-fixes@lists.freedesktop.org
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171208214616.30147-1-chris@chris-wilson.co.uk