drm/i915: Split sink_crc function in start, stop and read.

This is just a preparation patch to make clear what operation we
are performing. There is no functional change on the sink crc
logic.

hsw_disable_ips has been moved a bit further in the start function
to avoid disabling ips when sink crc is not going to be started.
and to avoid goto on this function.

v2: explain why hsw_disable_ips() call place has changed.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>