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

drivers/gpu/drm/ast/Makefile

@@ -3,7 +3,7 @@
 # Makefile for the drm device driver.  This driver provides support for the
 # Direct Rendering Infrastructure (DRI) in XFree86 4.1.0 and higher.
 
-ast-y := ast_cursor.o ast_drv.o ast_main.o ast_mode.o ast_ttm.o ast_post.o \
+ast-y := ast_cursor.o ast_drv.o ast_main.o ast_mm.o ast_mode.o ast_post.o \
 	 ast_dp501.o
 
 obj-$(CONFIG_DRM_AST) := ast.o