drm/i915/gt: Scrub execlists state on resume

Before we resume, we reset the HW so we restart from a known good state.
However, as a part of the reset process, we drain our pending CS event
queue -- and if we are resuming that does not correspond to internal
state. On setup, we are scrubbing the CS pointers, but alas only on
setup.

Apply the sanitization not just to setup, but to all resumes.

Reported-by: Venkata Ramana Nayana <venkata.ramana.nayana@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Venkata Ramana Nayana <venkata.ramana.nayana@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200416114117.3460-1-chris@chris-wilson.co.uk