drm/i915/fbc: Use PLANE_HAS_FENCE to determine if the plane is fenced

Rather than trusting the cached value of plane_state->vma->fence to
imply whether the plane_state itself holds a reference on the
framebuffer's fence, use the information provided in the
plane_state->flags (PLANE_HAS_FENCE). Note that we still assume that FBC
is entirely bounded by the plane_state active life span; it's not clear
if that is a safe assumption.

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/20180220134208.24988-4-chris@chris-wilson.co.uk