server: Initialise serial number counter for wl_display

This ends up propagating through and creating a valgrind error like:

==22573== Conditional jump or move depends on uninitialised value(s)
==22573==    at 0x409E57: pointer_attach (compositor.c:1999)
==22573==    by 0x34ECE05D63: ffi_call_unix64 (unix64.S:75)
==22573==    by 0x34ECE05784: ffi_call (ffi64.c:486)
==22573==    by 0x5674C45: wl_closure_invoke (connection.c:770)
==22573==    by 0x566ECCB: wl_client_connection_data (wayland-server.c:255)
==22573==    by 0x56722F1: wl_event_source_fd_dispatch (event-loop.c:79)
==22573==    by 0x5672C91: wl_event_loop_dispatch (event-loop.c:410)
==22573==    by 0x56705F4: wl_display_run (wayland-server.c:1003)
==22573==    by 0x40C775: main (compositor.c:2937)