drm: Roll out acquire context for the page_flip ioctl

Again just prep work.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170322215058.8671-11-daniel.vetter@ffwll.ch