drm/bridge: ti-tfp410: support hpd via gpio

On some boards the hpd pin of a hdmi connector is wired up to a gpio
pin. Since in the DRM world the tfp410 driver is responsible for
handling the connector, add support for hpd gpios in this very driver.

Reviewed-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Christopher Spinrath <christopher.spinrath@rwth-aachen.de>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/2e47786ab3d04078ae70d0c4064f7c4d@rwthex-s1-b.rwth-ad.de