drm/mgag200: Convert mgag200 driver to |struct drm_gem_vram_object|

The data structure |struct drm_gem_vram_object| and its helpers replace
|struct mgag200_bo|. It's the same implementation; except for the type
names.

v4:
	* cleanups from checkpatch.pl
	* select config option DRM_VRAM_HELPER

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20190508082630.15116-15-tzimmermann@suse.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>