fbdev: Use _SYSMEM_ infix for system-memory helpers

Change the infix for fbdev's system-memory helpers from _SYS_ to
_SYSMEM_. The helpers perform operations within system memory, but
not on the state of the operating system itself. Naming should make
this clear. Adapt all users. No functional changes.

Suggested-by: Helge Deller <deller@gmx.de>
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/20230729193157.15446-3-tzimmermann@suse.de