drm/bochs: Replace prepare_fb()/cleanup_fb() with GEM VRAM helpers

GEM VRAM provides an implementation for prepare_fb() and cleanup_fb()
of struct drm_simple_display_pipe_funcs. Switch over bochs.

v2:
	* use helpers for struct drm_simple_display_pipe_funcs

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191024081404.6978-3-tzimmermann@suse.de