drm/mgag200: Rename mgag200_ttm.c to mgag200_mm.c

The mgag200 driver does not use TTM any longer. Rename the related file
to mgag200_mm.c (as in 'memory management').

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200605135803.19811-5-tzimmermann@suse.de