drm/i915: Stop touching hangcheck.seqno from intel_engine_init_global_seqno()

The hangcheck runs independently to the main flow of seqno through the
driver. However, we have an odd coupling of the seqno reset that is
unwelcome, and if poked at just the right rate can cause spurious hangs
(e.g. gem_exec_whisper) on an apparently idle engine.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170421083113.21321-1-chris@chris-wilson.co.uk