drm: Add explicit acquire ctx handling around ->set_config

Just the groundwork to have something to feed into ->set_config.
Again we need a temporary hack to still fill out the legacy
ctx in mode_config.acquire_ctx.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170322215058.8671-18-daniel.vetter@ffwll.ch