drm: check for garbage in unused addfb2 fields

Unfortunately old userspace didn't clear this properly, but since
we've added fb modifiers that's fixed. Checking properly that unused
fields is important for abi extensions, and just right now there's a
bunch of discussions going on about how exactly the additional aux
planes for render compression should be specified.

So let's first make sure that the values in those additional fields
can be indeed used.

Cc: Thierry Reding <thierry.reding@gmail.com>
Testcase: igt/kms_addfb/unused-*
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>