Add tracepoints for display FIFO underruns. Makes it more convenient to
correlate the underruns with other display tracepoints.

v2: s/i915/intel/ in the tracepoint name
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/20170302171508.1666-19-ville.syrjala@linux.intel.com