drm/i915/gt: Always reschedule the new heartbeat

In order to better respond to new heartbeat intervals given via sysfs,
always reprogramme an active heartbeat upon change (i.e. use
mod_delayed_work to reschedule rather than queue_delayed_work which
ignores an already active work.)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200317163208.30010-1-chris@chris-wilson.co.uk