freedreno: constify fd_tile

In a following patch, when we cache the gmem state, we will want to
treat the gmem state as immuatable.  So start converting things to
const to make this more clear.. fd_tile is a good place to start.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3503>