drm/bridge: ti-tfp410: switch to using fwnode_gpiod_get_index()

Instead of fwnode_get_named_gpiod() that I plan to hide away, let's use
the new fwnode_gpiod_get_index() that mimics gpiod_get_index(), but
works with arbitrary firmware node.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>