drm/rockchip: add extcon dependency for DP

The newly added DP driver links against the extcon core, which fails when
extcon is a module and this driver is not:

drivers/gpu/drm/rockchip/cdn-dp-core.o: In function `cdn_dp_get_port_lanes':
cdn-dp-core.c:(.text.cdn_dp_get_port_lanes+0x24): undefined reference to `extcon_get_state'
cdn-dp-core.c:(.text.cdn_dp_get_port_lanes+0x44): undefined reference to `extcon_get_property'

Let's make Kconfig enforce correct behavior with a dependency.

Fixes: 1a0f7ed3

 ("drm/rockchip: cdn-dp: add cdn DP support for rk3399")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Acked-by: Mark Yao <mark.yao@rock-chips.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170214213215.2888509-1-arnd@arndb.de