drm/udl: Replace struct udl_framebuffer with generic implementation

The udl driver's struct udl_framebuffer stores a DRM framebuffer
with an associated GEM object. This functionality is also provided by
generic code. Switch udl over.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191114141025.32198-6-tzimmermann@suse.de