drm/i915/fbdev: Use the PLANE_HAS_FENCE flags from the time of pinning

Use the information about the fence state from the time of pinning to
determine if the fbdev writes are going through a fence. This avoids any
confusion in cases where the fence may appear or disappear unconnected
to the use by fbdev.

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180220134208.24988-3-chris@chris-wilson.co.uk