fbdev: Rename FB_SYS_FOPS token to FB_SYSMEM_FOPS

Rename the token to harmonize naming among various helpers. For
example, I/O-memory helpers use FB_IOMEM_FOPS.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231127131655.4020-28-tzimmermann@suse.de