drm: bridge: add support for TI ths8135

THS8135 is a configurable video DAC, but no configuration is actually
necessary to make it work.

For now use the dumb-vga-dac driver to support it.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1481623759-12786-4-git-send-email-bgolaszewski@baylibre.com