drm/omap: displays: Don't call disconnect handlers directly

In preparation for the move of checks from the disconnect handlers to
the omapdss_device_disconnect() function, replace direct calls to the
disconnect handlers at remove time with calls to
omapdss_device_disconnect().

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>