drm/atomic: Move __drm_atomic_helper_disable_plane/set_config()

Prepare for moving drm_fb_helper modesetting code to drm_client.
drm_client will be linked to drm.ko, so move
__drm_atomic_helper_disable_plane() and __drm_atomic_helper_set_config()
out of drm_kms_helper.ko.

While at it, fix two checkpatch complaints:
- WARNING: Block comments use a trailing */ on a separate line
- CHECK: Alignment should match open parenthesis

v7: Declare drm_mode_set and drm_plane_state

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190531140117.37751-2-noralf@tronnes.org