drm/i915: Simplify reset_counter handling during atomic modesetting

Now that the reset_counter is stored on the request, we can rearrange
the code to handle reading the counter versus waiting during the atomic
modesetting for readibility (by deleting the hairiest of codes).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1460565315-7748-8-git-send-email-chris@chris-wilson.co.uk