drm/bochs: Use managed initialization for GEM VRAM helpers

Convert to managed GEM VRAM initialization and switch bochs to
full autocleanup.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210702075434.27677-3-tzimmermann@suse.de