drm/panfrost: Add an helper to check the GPU generation

All models with an ID >= 0x1000 are Bifrost GPUs for now (might change
with new gens).

Suggested-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190618081648.17297-4-boris.brezillon@collabora.com