drm/display: Select DRM_KMS_HELPER for DP helpers

The DisplayPort helpers rely on some
(__drm_atomic_helper_private_obj_duplicate_state,
drm_kms_helper_hotplug_event) helpers found in files compiled by
DRM_KMS_HELPER.

Prior to commit d674858f ("drm/display: Make all helpers visible and
switch to depends on"), DRM_DISPLAY_DP_HELPER was only selectable so it
wasn't really a big deal. However, since that commit, it's now something
that can be enabled as is, and since there's no expressed dependency
with DRM_KMS_HELPER, it can break too.

Since DRM_KMS_HELPER is a selectable option for now, let's select it for
DRM_DISPLAY_DP_HELPER.
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202404021556.0JVcNC13-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202404021700.LbyYZGFd-lkp@intel.com/
Fixes: d674858f ("drm/display: Make all helpers visible and switch to depends on")
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240403-fix-dw-hdmi-kconfig-v1-1-afbc4a835c38@kernel.org

drivers/gpu/drm/display/Kconfig

@@ -39,6 +39,7 @@ config DRM_DISPLAY_DP_AUX_CHARDEV
 config DRM_DISPLAY_DP_HELPER
 	bool "DRM DisplayPort Helpers"
 	depends on DRM_DISPLAY_HELPER
+	select DRM_KMS_HELPER
 	default y
 	help
 	  DRM display helpers for DisplayPort.