drm: change CONFIG_DRM_KMS_HELPER to =m

This option is m selected by our dependencies, so lets align with
them and avoid a warning.

We keep the option, so that if it changes in the future, we'll get
the warning again.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>

features/drm-gma500/drm-gma500.cfg

 # SPDX-License-Identifier: MIT
 CONFIG_DRM=y
-CONFIG_DRM_KMS_HELPER=y
+CONFIG_DRM_KMS_HELPER=m
 CONFIG_DRM_GMA500=m
 CONFIG_DRM_GMA600=y
 CONFIG_DRM_GMA3600=y