drm/i915/fbc: call intel_fbc_pre_update earlier during page flips

Make sure we do the pre_update - which also deactivates FBC - before
we actually schedule the page flip, just to make sure we don't
flip to the new FB with FBC still activated for the previous FB.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453210558-7875-24-git-send-email-paulo.r.zanoni@intel.com