A bug that I had fixed earlier just came back, with CONFIG_EXTCON=m,
the rockchip drm driver will fail to link:

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+0x30): undefined reference to `extcon_get_state'
cdn-dp-core.c:(.text.cdn_dp_get_port_lanes+0x6c): undefined reference to `extcon_get_property'
drivers/gpu/drm/rockchip/cdn-dp-core.o: In function `cdn_dp_check_sink_connection':
cdn-dp-core.c:(.text.cdn_dp_check_sink_connection+0x80): undefined reference to `extcon_get_state'
drivers/gpu/drm/rockchip/cdn-dp-core.o: In function `cdn_dp_enable':
cdn-dp-core.c:(.text.cdn_dp_enable+0x748): undefined reference to `extcon_get_property'

The problem is that that the sub-drivers are now all linked into the
main rockchip drm module, which breaks all the Kconfig dependencies
that are specified in the options for those sub-drivers.

This clarifies the dependency to ensure that we can only turn on the DP
driver when EXTCON is reachable. As the 'select' statements can now
cause additional options to become built-in when they should be
loadable modules, I'm moving those into the main driver config option.
The dependency on DRM_ROCKCHIP can be reduced into a single 'if'
statement here for brevity, but this has no functional effect.

Fixes: b6705157 ("drm/rockchip: add extcon dependency for DP")
Fixes: 8820b68b ("drm/rockchip: Refactor the component match logic.")
Link: https://patchwork.kernel.org/patch/9648761/Acked-by: Guenter Roeck <groeck@chromium.org>
Tested-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170721211214.3386387-1-arnd@arndb.de