fbdev: Remove constants of unused events

The constants FB_EVENT_MODE_CHANGE and FB_EVENT_BLANK are unused.
Remove them from the header file.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://lore.kernel.org/r/20250321095517.313713-12-tzimmermann@suse.de


Signed-off-by: Lee Jones <lee@kernel.org>