backend-drm: add config option require-outputs

If Weston fails to configure a DRM output for whatever reason, it will fail to
start. Depending on the use-case, this may or may not be the correct behavior.

Add the "require-outputs" option to allow configuring the error behavior on
missing outputs.

Add documentation of the possible options to the man page.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>