drm/i915/execlists: Double check breadcrumb before crying foul

  process_csb: 0000:00:02.0 bcs0: cs-irq head=4, tail=5
  process_csb: 0000:00:02.0 bcs0: csb[5]: status=0x00008002:0x60000020
  trace_ports: 0000:00:02.0 bcs0: preempted { ff84:45154! prio 2 }
  trace_ports: 0000:00:02.0 bcs0: promote { ff84:45155* prio 2 }
  trace_ports: 0000:00:02.0 bcs0: submit { ff84:45156 prio 2 }

  process_csb: 0000:00:02.0 bcs0: cs-irq head=5, tail=6
  process_csb: 0000:00:02.0 bcs0: csb[6]: status=0x00000018:0x60000020
  trace_ports: 0000:00:02.0 bcs0: completed { ff84:45155* prio 2 }
  process_csb: 0000:00:02.0 bcs0: ring:{start:0x00178000, head:0928, tail:0928, ctl:00000000, mode:00000200}
  process_csb: 0000:00:02.0 bcs0: rq:{start:00178000, head:08b0, tail:08f0, seqno:ff84:45155, hwsp:45156},
  process_csb: 0000:00:02.0 bcs0: ctx:{start:00178000, head:e000928, tail:0928},
  process_csb: GEM_BUG_ON("context completed before request")

In this sequence, we can see that although we have submitted the next
request [ff84:45156] to HW (via ELSP[]) it has not yet reported the
lite-restore. Instead, we see the completion event of the currently
active request [ff84:45155] but at the time of processing that event,
the breadcrumb has not yet been written. Though by the time we do print
out the debug info, the seqno write of ff84:45156 has landed!

Therefore there is a serialisation problem between the seqno writes and
CS events, not just between the CS buffer and its head/tail pointers as
previously observed on Icelake.

This is not a huge problem, as we don't strictly rely on the breadcrumb
to determine HW activity, but it may indicate that interrupt delivery is
before the seqno write, aka bringing back the plague of missed
interrupts from yesteryear. However, there is no indication of this
wider problem, so let's just flush the seqno read before reporting an
error. If it persists after the fresh read we can worry again.

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/20200330234318.30638-1-chris@chris-wilson.co.uk