drm/i915: remove unneeded visible check

The fb check introduced to drm_plane_helper_check_update() just make this
check impossible to branch in.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>