drm/i915/fbc: Store fence_id directly in fbc cache/params

Rather than playing around with vma+flags let's just grab
the fence id from within and stash that directly in the fbc
cache/params.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127201222.16669-8-ville.syrjala@linux.intel.com


Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>