Revert "drm/bridge: Add an ->atomic_check() hook"

This reverts commit b86d8955 ("drm/bridge: Add an ->atomic_check()
hook"). Commit 6ed7e962

 ("drm/bridge: Add a drm_bridge_state
object") introduced a circular dependency between drm.ko and
drm_kms_helper.ko which uncovered a misdesign in how the whole thing
was implemented. Let's revert all patches depending on the bridge_state
infrastructure for now.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200107185807.606999-4-boris.brezillon@collabora.com