drm: Remove bridge support from legacy helpers

DRM bridges are only used by atomic drivers, and none of them use the
legacy helpers. Drop bridge support from those helpers to prepare for
making the bridge operations atomic-aware.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>