drm/i915: Fix ILK-IVB primary plane enable delays

Primary and sprite plane enable on ILK-IVB may take two frames to complete

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103925


Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1545305168-6047-1-git-send-email-juhapekka.heikkila@gmail.com