drm: add some caps for userspace to discover more info for dumb KMS driver (v2)

For the simple KMS driver case we need some more info about what the preferred
depth and if a shadow framebuffer is preferred.

I've only added this for intel/radeon which support the dumb ioctls so far.

If you need something really fancy you should be writing a real X.org driver.

v2: drop cursor information, just return an error from the cursor ioctls
and we can make userspace fallback to sw cursor in that case, cursor
info was getting too messy, best to start smaller.

Signed-off-by: Dave Airlie <airlied@redhat.com>