drm/client: Add drm_client_modeset_check()

Add a way for client to check the configuration before comitting.

v2:
- Fix docs (Sam)

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200509141619.32970-5-noralf@tronnes.org