drm/i915: Move the irq_barrier for reset earlier into reset_prepare

drm/i915: Move the irq_barrier for reset earlier into reset_prepare

When updating the bookkeeping following the reset, we need the seqno to
be coherent on the CPU prior to trusting its result for deciding whether
any request is completed. We need the irq_barrier before we start making
these decisions, i.e. in reset_prepare.

References: https://bugs.freedesktop.org/show_bug.cgi?id=99733
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170210185214.23463-1-chris@chris-wilson.co.uk
Reviewed-by: Michel Thierry <michel.thierry@intel.com>