drm/i915/gt: Track the most recent pulse for the heartbeat

Since we track the idle_pulse for flushing the barriers and avoid
re-emitting the pulse upon idling if no futher action is required, this
also impacts the heartbeat. Before emitting a fresh heartbeat, we look
at the engine idle status and assume that if the pulse was the last
request emitted along the heartbeat, the engine is idling and a
heartbeat pulse not required. This assumption fails, but we can reuse
the idle pulse as the heartbeat if we are yet to emit one, and so track
the status of that pulse for our engine health check.

This impacts tgl/rcs0 as we rely on the heartbeat for our healthcheck for
the normal preemption detection mechanism is disabled by default.

Testcase: igt/gem_exec_schedule/preempt-hang/rcs0 #tgl
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201006094653.7558-1-chris@chris-wilson.co.uk