compositor-drm: get current mode on head discovery

The inherited mode is the video mode on the connector when we have not
yet reconfigured the connector, if set.

Get the inherited mode the moment we create a drm_head, not when we
determine the mode for a drm_output. This way we are sure to read all
inherited modes before we reconfigure a single CRTC. Enabling one output
may grab the CRTC from another connector, overwriting whatever mode that
connector might have had.

The inherited mode is stored in drm_head, where we can keep it for the
lifetime of the head, rather than relying on re-loading it from the
kernel at set_mode() time.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>