panfrost: Add padded type for instance fields

panfrost: Add padded type for instance fields

Mali has a special 5:3 encoding representing a subset of the natural
numbers, of the form:

   a * (2^b)

for a odd and b natural/zero. It is used for padding out instance sizes,
as well as in attribute records so it's worth representing as a native
type as opposed to having manual packs/unpacks in various places.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6476>