drm/atomic-helper: Remove the backoff hack from set_config

Another one bites the dust.

Again let's not forget to remove the temporary hidden acquire_ctx
assignment, now that we pass this all around explicitly it can go
away again.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170322215058.8671-20-daniel.vetter@ffwll.ch