drm/i915: Wait for vblank after cxsr disable in pre_plane_update

drm/i915: Wait for vblank after cxsr disable in pre_plane_update

We must wait for the hardware to exit cxsr before doing the plane
update, so add the missing vblank wait to pre_plane_update after
disabling cxsr.

We have the wait for vblank in the pre_disable_primary hook, but not in
the pre_plane_update hook. Just move the code from (and comment) from
pre_disable_primary into pre_plane_update. Well, we still have to keep
it in pre_disable_primary for these strange _noatomic codepaths, so
let's do another version of pre_disable_primary for those. Also toss
in some FIXMEs in the hope that someone will eventually clean up this
pre_disable_primary mess.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1457543247-13987-5-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>