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

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

So that bridge drivers have a way to check/reject an atomic operation.
The drm_atomic_bridge_chain_check() (which is just a wrapper around
the ->atomic_check() hook) is called in place of
drm_bridge_chain_mode_fixup() (when ->atomic_check() is not implemented,
the core falls back on ->mode_fixup(), so the behavior should stay
the same for existing bridge drivers).

Signed-off-by: Boris Brezillon <[email protected]>
Reviewed-by: Neil Armstrong <[email protected]>
Reviewed-by: Laurent Pinchart <[email protected]>
Signed-off-by: Neil Armstrong <[email protected]>
Reviewed by: Jernej Skrabec <[email protected]>
Tested-by: Jonas Karlman <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]