drm/i915: Call intel_fbc_pre_update() after pinning the new pageflip

intel_fbc_pre_update() depends upon the new state being already pinned
in place in the Global GTT (primarily for both fencing which wants both
an offset and a fence register, if assigned). This requires the call to
intel_fbc_pre_update() be after intel_pin_and_fence_fb() - but commit
e8216e50 ("drm/i915/fbc: call intel_fbc_pre_update earlier during
page flips") moved the code way too much up in its attempt to call it
before the page flip.

v2 (from Paulo):
 - Point the original bad commit.
 - Add a comment to maybe prevent further regressions.

Fixes: e8216e50 ("drm/i915/fbc: call intel_fbc_pre_update earlier...")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Cc: drm-intel-fixes@lists.freedesktop.org
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1471462904-842-1-git-send-email-paulo.r.zanoni@intel.com
Cc: stable@vger.kernel.org