drm/i915: Add VLV/CHV watermark/FIFO programming tracepoints

Add tracepoints for observing the WM/FIFO programming on VLV/CHV. When
compared with the plane and pipe update tracepoints this can be used
to verify that everything is performed in the right sequence.

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/20170302171508.1666-17-ville.syrjala@linux.intel.com