freedreno/ir3: un-embed const_state

freedreno/ir3: un-embed const_state

Make it an rzalloc'd ptr instead of embedded struct, so it can serve as
the mem ctx for immediates.  This gets rid of needing to explicitly free
the immediates, so one less thing to deal with when moving const_state.
(Also, after we move const_state to the shader variant, we won't need
one for binning pass variants)

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