drm/mgag200: Convert mgag200 driver to VRAM MM

The data structure |struct drm_vram_mm| and its helpers replace mgag200's
TTM-based memory manager. It's the same implementation; except for the type
names.

v4:
	* don't select DRM_TTM or DRM_VRAM_MM_HELPER
v3:
	* use drm_gem_vram_mm_funcs
	* convert driver to drm_device-based instance
v2:
	* implement mgag200_mmap() with drm_vram_mm_mmap()

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20190508082630.15116-16-tzimmermann@suse.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>