The ast driver's struct ast_framebuffer is a buffer object with GEM
interface. There are already GEM framebuffer helpers that implement
the same functionality. Convert ast to these.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190627173410.8300-1-tzimmermann@suse.de