fbdev: Use _DMAMEM_ infix for DMA-memory helpers

Change the infix for fbdev's DMA-memory helpers from _DMA_ to
_DMAMEM_. The helpers perform operations within DMA-able memory,
but they don't perform DMA operations. Naming should make this
clear. Adapt all users. No functional changes.

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/20230729193157.15446-4-tzimmermann@suse.de