drm/i915: Extract intel_wm_plane_visible()

drm/i915: Extract intel_wm_plane_visible()

All platforms that lack double buffered watermarks will need to
handle the legacy cursor updates in the same way. So let's extract the
logic to determine the plane visibility into a small helper. For
simplicity we'll make the function DTRT for any plane, but only apply
the special sauce for cursor planes.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170314151050.12194-1-ville.syrjala@linux.intel.com
Tested-by: Dorota Czaplejewicz <dorota.czaplejewicz@collabora.co.uk>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>