compositor-drm: Fix crash when setting up seat constrained by an output

compositor-drm: Fix crash when setting up seat constrained by an output

Commit 58e15865 changed the parameters for udev_get_seat_by_name() to
receive a struct udev_input. However, when this gets called from
create_output_from_connector() during initialization, the input struct
is not yet initialized, leading to a crash. Previously, that function
would take only a pointer to the compositor.

This patch fixes the crash by initializing input before creating any
outputs.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=77503