drm/i915/execlists: Leave resetting ring to intel_ring

We need to allow concurrent intel_context_unpin, which means avoiding
doing destructive operations like intel_ring_reset(). This was already
fixed for intel_ring_unpin() in commit 0725d9a3 ("drm/i915/gt: Make
intel_ring_unpin() safe for concurrent pint"), but I overlooked that
execlists_context_unpin() also made the same mistake.

Reported-by: Matthew Brost <matthew.brost@intel.com>
Fixes: 84135022 ("drm/i915/gt: Drop mutex serialisation between context pin/unpin")
References: 0725d9a3 ("drm/i915/gt: Make intel_ring_unpin() safe for concurrent pint")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200115175829.2761329-1-chris@chris-wilson.co.uk