drm/i915: Reset the hangcheck timestamp before repeating a seqno

In the unusual circumstance where we reuse a seqno (for example, in
igt), make sure that we reset the hangcheck timestamp before it sees the
same seqno again.

References: https://bugs.freedesktop.org/show_bug.cgi?id=106215


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180502220313.6459-1-chris@chris-wilson.co.uk