fbdev/metronomefb: Select FB_SYS_HELPERS_DEFERRED

The Kconfig token FB_SYS_HELPERS_DEFERRED selects everything that
is required for deferred I/O on system-memory framebuffers. Select
it from FB_METRONOME in favor of the existing identical selection.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230706151432.20674-6-tzimmermann@suse.de