drm/bridge: ti-sn65dsi86: Don't compile GPIO bits if not CONFIG_OF_GPIO

The kernel test robot noted that if "OF" is defined (which is needed
to select DRM_TI_SN65DSI86 at all) but not OF_GPIO that we'd get
compile failures because some of the members that we access in "struct
gpio_chip" are only defined "#if defined(CONFIG_OF_GPIO)".

All the GPIO bits in the driver are all nicely separated out.  We'll
guard them with the same "#if defined" that the header has and add a
little stub function if OF_GPIO is not defined.

Fixes: 27ed2b3f

 ("drm/bridge: ti-sn65dsi86: Export bridge GPIOs to Linux")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200612123003.v2.1.Ibe95d8f3daef01e5c57d4c8c398f04d6a839492c@changeid