drm/i915: Make legacy cursor updates more unsynced

We're clearing the legacy_cursor_update flag before calling
drm_atomic_helper_setup_commit() which means the helper will
wait for the flip to complete before cleaning up the framebuffers.
That's not what we want for the legacy cursor, so let's clear
the flag after setting up the commit.

Also toss in a FIXME about solving these problems in a nicer
way using the fabled vblank workers.

v2: Also unsync with legacy page flips

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Uwe Kleine-König <uwe@kleine-koenig.org>
Cc: Rafael Ristovski <rafael.ristovski@gmail.com>
Fixes: a5509abd

 ("drm/i915: Fix legacy cursor vs. watermarks for ILK-BDW")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170329142123.5923-1-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>