drm/i915: Add cxsr toggle tracepoint

Add a tracepoint for observing changes in the cxsr state. The tracepoint
will dump out the frame and scanline counters for each pipe so that the
information can be compared with eg. plane update tracepoints.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170302171508.1666-18-ville.syrjala@linux.intel.com