drm-backend: add --continue-without-input command line option to DRM-backend

drm-backend: add --continue-without-input command line option to DRM-backend

In the test suite we may want to run a DRM-backend test on a
non-default seat, which may not have a input device associated.
Weston's default behavior is to not open if input devices are
not found, as it may cause troubles. For instance, Weston can
open but if no input device is set than the user can not
interact or leave it.

Add flag --continue-without-input to DRM-backend so we can run
these types of tests with no input. Notice that this won't force
the compositor to skip opening a input device if it finds it on
the non-default seat.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>