freedreno/ir3: add accessor for const_state

freedreno/ir3: add accessor for const_state

We are going to want to move this back to the variant, and come up with
a different strategy for binning/nonbinning to share the same constant
layout, in order to implement shader-cache support.  (Since then we
can have a mix of dynamically compiled variants and cache hits, so there
is no good place to serialize the const-state.)

To reduce the churn as we re-arrange things, move direct access to the
const-state to a helper fxn.  This patch is the boring churny part.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5508>