Documentation: Fix warning in drm-kms-helpers.rst

Documentation: Fix warning in drm-kms-helpers.rst

Fixes the following warning:
../include/drm/drm_atomic_state_helper.h:1: warning: no structured comments found

Fixes: 9ef8a9dc4b21 ("drm: Extract drm_atomic_state_helper.[hc]")
Cc: Ville Syrjälä <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Maxime Ripard <[email protected]>
Cc: Sean Paul <[email protected]>
Cc: David Airlie <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: [email protected]
Reviewed-by: Daniel Vetter <[email protected]>
Signed-off-by: Sean Paul <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]