drm/core: Add drm_afbc_framebuffer and a corresponding helper

The new struct contains afbc-specific data.

The new function can be used by drivers which support afbc to complete
the preparation of struct drm_afbc_framebuffer. It must be called after
allocating the said struct and calling drm_gem_fb_init_with_funcs().

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: James Qian Wang <james.qian.wang@arm.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200311145541.29186-3-andrzej.p@collabora.com