drm/i915: Add g4x watermark tracepoint

Add a tracepoint for watermark programming on g4x, similar to what we
have on vlv/chv. Should help in debugging watermark programming sequence
issues.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170421181432.15216-15-ville.syrjala@linux.intel.com


Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>