radv: Set correct metadata size for GFX9+.

Without correct size, radeonsi assumes the metadata is incorrect,
which can and will cause issues.

Since the metadata is really incorrect without the size, let us
fix that.

Fixes: e43cc3e3 "radv/gfx9: handle GFX9 opaque metadata"
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit 7e1fe81f)