modetest: Add a new "-r" option to set a default mode

This option finds all connected connector and then sets its preferred
mode on it. If no preferred mode is available, first mode is used.

This option must be set w/o any mode or plane.

This allows for a quick test on all connected outputs.

Loosely based on the work by Ezequiel Garcia <ezequiel@collabora.com>

Changes since Ezequiel's work:
 - implement atomic codepath
 - set all connectors
 - pick correct crtc
 - don't set -r by default
 - nearly identical output in atomic and non-atomic codepaths

v2:
 - Use the crtc->crtc_id, instead of the plane's current crtc_id

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
Tested-by: Ezequiel Garcia <ezequiel@collabora.com>