drm/ast: Rename ast_ttm.c to ast_mm.c

Although built upon TTM, the ast driver's VRAM MM helper does not
expose TTM to its users. Rename the related ast file to ast_mm.c.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200716125353.31512-3-tzimmermann@suse.de