drm/bridge: ite-it6505: add missing Kconfig option select

drm/bridge: ite-it6505: add missing Kconfig option select

The IT6505 is using functions provided by the DRM_DP_HELPER driver.
In order to avoid having the bridge enabled but the helper disabled,
let's add a select in order to be sure that the DP helper functions are
always available.

Fixes: d9992fae4dbc ("drm/bridge: add it6505 driver")
Signed-off-by: Fabien Parent <fparent@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220426141536.274727-1-fparent@baylibre.com