drm/cirrus: cirrus_framebuffer -> drm_framebuffer

Now cirrus_framebuffer is just an empty wrapper around drm_framebuffer,
we can drop it.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Link: https://patchwork.freedesktop.org/patch/msgid/20180330141138.28987-2-daniels@collabora.com