drm/bridge: display-connector: Select DRM_KMS_HELPER

Commit 7cd70656 ("drm/bridge: display-connector: implement
bus fmts callbacks") added use of drm_atomic_helper_bridge_*
functions, but didn't select the dependency of DRM_KMS_HELPER.
If nothing else selected that dependency it resulted in a
build failure.

Select the missing dependency.

Fixes: 7cd70656

 ("drm/bridge: display-connector: implement bus fmts callbacks")
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>