fbdev/core: Clean up include statements in fbmem.c

Remove all unnecessary include statements from fbmem.c. Most of
them were for functionality that has meanwhile been moved into
other files.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230907085408.9354-8-tzimmermann@suse.de