drm/bridge: fix LONTIUM_LT8912B dependencies

LONTIUM_LT8912B uses "drm_display_mode_to_videomode" from
DRM framework that needs VIDEOMODE_HELPERS to be enabled.

Fixes: 30e2ae94 ("drm/bridge: Introduce LT8912B DSI to HDMI bridge")
Reported-by: Michal Suchánek <msuchanek@suse.de>
Signed-off-by: Adrien Grassein <adrien.grassein@gmail.com>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210504220207.4004511-1-adrien.grassein@gmail.com