drm: Don't make DRM_PANEL_BRIDGE dependent on DRM_KMS_HELPERS

Fix a number of undefined references to drm_kms_helper.ko in
drm_dp_helper.ko:

  arm-suse-linux-gnueabi-ld: drivers/gpu/drm/dp/drm_dp_mst_topology.o: in function `drm_dp_mst_duplicate_state':
  drm_dp_mst_topology.c:(.text+0x2df0): undefined reference to `__drm_atomic_helper_private_obj_duplicate_state'
  arm-suse-linux-gnueabi-ld: drivers/gpu/drm/dp/drm_dp_mst_topology.o: in function `drm_dp_delayed_destroy_work':
  drm_dp_mst_topology.c:(.text+0x370c): undefined reference to `drm_kms_helper_hotplug_event'
  arm-suse-linux-gnueabi-ld: drivers/gpu/drm/dp/drm_dp_mst_topology.o: in function `drm_dp_mst_up_req_work':
  drm_dp_mst_topology.c:(.text+0x7938): undefined reference to `drm_kms_helper_hotplug_event'
  arm-suse-linux-gnueabi-ld: drivers/gpu/drm/dp/drm_dp_mst_topology.o: in function `drm_dp_mst_link_probe_work':
  drm_dp_mst_topology.c:(.text+0x82e0): undefined reference to `drm_kms_helper_hotplug_event'

This happens if panel-edp.ko has been configured with

  DRM_PANEL_EDP=y
  DRM_DP_HELPER=y
  DRM_KMS_HELPER=m

which builds DP helpers into the kernel and KMS helpers sa a module.
Making DRM_PANEL_EDP select DRM_KMS_HELPER resolves this problem.

To avoid a resulting cyclic dependency with DRM_PANEL_BRIDGE, don't
make the latter depend on DRM_KMS_HELPER and fix the one DRM bridge
drivers that doesn't already select DRM_KMS_HELPER. As KMS helpers
cannot be selected directly by the user, config symbols should avoid
depending on it anyway.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 3755d35e

 ("drm/panel: Select DRM_DP_HELPER for DRM_PANEL_EDP")
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Tested-by: Brian Masney <bmasney@redhat.com>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Naresh Kamboju <naresh.kamboju@linaro.org>
Cc: Linux Kernel Functional Testing <lkft@linaro.org>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: dri-devel@lists.freedesktop.org
Cc: Dave Airlie <airlied@redhat.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Link: https://patchwork.freedesktop.org/patch/478296/