drm: crc: Wait for a frame before returning from open()

Don't return from the open() call on the crc/data file until the HW has
produced a first frame, as there's great variability in when the HW is
able to do that and userspace shouldn't have to guess when this specific
HW is ready to start giving frame CRCs.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170102125912.22305-3-tomeu.vizoso@collabora.com