compositor-drm: update video mode printing

Stop using a head for printing the mode list, because there could be
multiple heads. We already gather the mode list from all heads.

No need to print the connector id here, because it is logged with DRM
heads, and core prints the head names on output enable.

The "built-in" flag seemed dead, because it could only be printed if the
kernel provided no modes. If we want more detailed info on where modes
come from, we would need to inspect mode_info or add new flags to
drm_mode or weston_mode.

Add printing the pixel clock, because that is used by the video mode
duplicate removal code.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derekf@osg.samsung.com>