drm/i915/gt: Drop a defunct timeline assertion

intel_timeline_enter() has been decoupled from intel_timeline_pin() and
both enter/exit & pin/unpin are allowed [read expected] to run
concurrently with one another. The assertion that they had better not is
stale.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/940
References: a6edbca7 ("drm/i915/gt: Close race between engine_park and intel_gt_retire_requests")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200107143826.3298401-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_timeline.c

@@ -348,7 +348,6 @@ void intel_timeline_enter(struct intel_timeline *tl)
 	 * use atomic to manipulate tl->active_count.
 	 */
 	lockdep_assert_held(&tl->mutex);
-	GEM_BUG_ON(!atomic_read(&tl->pin_count));
 
 	if (atomic_add_unless(&tl->active_count, 1, 0))
 		return;