drm: Wire up proper acquire ctx for plane functions

This is just prep work to get an acquire ctx into every place where we
call ->update_plane or ->disable_plane.

v2: Keep the hidden acquire_ctx pointers valid while transitioning.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170322215058.8671-2-daniel.vetter@ffwll.ch