drm/i915: Fix misplaced intel_mark_page_flip_active()

In commit 28c4566d30b8, backport of commit e7d841ca03b7 ('drm/i915:
Close race between processing unpin task and queueing the flip') I
somehow added two calls to intel_mark_page_flip_active() from
intel_gen4_queue_flip() and none from intel_gen6_queue_flip().  There
should of course be one from each.

Reported-by: Julien Cristau <jcristau@debian.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>