drm/i915/selftests: Bump the scheduling error threshold for fast heartbeats

Since we are system_highpri_wq, we expected the heartbeat to be
scheduled promptly. However, we see delays of over 10ms upsetting our
assertions. Accept this as inevitable and bump the minimum error
threshold to 20ms (from 6 jiffies).

<6> [616.784749] rcs0: Heartbeat delay: 3570us [2802, 9188]
<6> [616.807790] bcs0: Heartbeat delay: 2111us [745, 4372]
<6> [616.853776] vcs0: Heartbeat delay: 6485us [2424, 11637]
<3> [616.859296] vcs0: Heartbeat delay was 6485us, expected less than 6000us
<3> [616.860901] i915/intel_heartbeat_live_selftests: live_heartbeat_fast failed with error -22

v2: More context from CI.

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: https://patchwork.freedesktop.org/patch/msgid/20210113163115.5740-2-chris@chris-wilson.co.uk