drm/i915/execlists: Ignore lost completion events

Icelake hit an issue where it missed reporting a completion event and
instead jumped straight to a idle->active event (skipping over the
active->idle and not even hitting the lite-restore preemption).

661497511us : process_csb: rcs0 cs-irq head=11, tail=0
661497512us : process_csb: rcs0 csb[0]: status=0x10008002:0x00000020 [lite-restore]
661497512us : trace_ports: rcs0: preempted { 28cc8:11052, 0:0 }
661497513us : trace_ports: rcs0: promote { 28cc8:11054, 0:0 }
661497514us : __i915_request_submit: rcs0 fence 28cc8:11056, current 11052
661497514us : __execlists_submission_tasklet: rcs0: queue_priority_hint:-2147483648, submit:yes
661497515us : trace_ports: rcs0: submit { 28cc8:11056, 0:0 }
661497530us : process_csb: rcs0 cs-irq head=0, tail=1
661497530us : process_csb: rcs0 csb[1]: status=0x10008002:0x00000020 [lite-restore]
661497531us : trace_ports: rcs0: preempted { 28cc8:11054!, 0:0 }
661497535us : trace_ports: rcs0: promote { 28cc8:11056, 0:0 }
661497540us : __i915_request_submit: rcs0 fence 28cc8:11058, current 11054
661497544us : __execlists_submission_tasklet: rcs0: queue_priority_hint:-2147483648, submit:yes
661497545us : trace_ports: rcs0: submit { 28cc8:11058, 0:0 }
661497553us : process_csb: rcs0 cs-irq head=1, tail=2
661497553us : process_csb: rcs0 csb[2]: status=0x10000001:0x00000000 [idle->active]
661497574us : process_csb: process_csb:1538 GEM_BUG_ON(*execlists->active)

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/20190907084334.28952-1-chris@chris-wilson.co.uk