drm/bridge: PTN3460 needs DRM_KMS_HELPER

The recently added PTN3460 device driver uses interfaces that
are provided by the KMS helper infrastructure, so we should
explicitly select that to avoid this linker error:

ERROR: "drm_helper_probe_single_connector_modes" [drivers/gpu/drm/bridge/ptn3460.ko] undefined!
ERROR: "drm_helper_connector_dpms" [drivers/gpu/drm/bridge/ptn3460.ko] undefined!

We have to drop the I2C dependency to avoid a circular dependency
chain, but that's ok because DRM already selects I2C.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>