anv: Remove special allocation for anv_push_constants

The key reason for that mechanism is gone: all the extra optional data
that could be in the anv_push_constants was moved elsewhere.  At this
point, just put anv_push_constants directly in anv_cmd_state (part of
anv_cmd_buffer).

v2: Remove a NULL check we don't need anymore in
    anv_cmd_buffer_push_constants().  (Lionel)
    Fix size we consider for valid push params.  (Lionel)

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
(cherry picked from commit f7d53fff)