drm: Link several object files into drm_kms_helper.ko

Several core DRM functions are not used by the DRM core. Link the
object files into the KMS helper library.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211020131941.15367-3-tzimmermann@suse.de