anv: Move push constant allocation to the command buffer

anv: Move push constant allocation to the command buffer

Instead of blasting it out as part of the pipeline, we put it in the
command buffer and only blast it out when it's really needed.  Since the
PUSH_CONSTANT_ALLOC commands aren't pipelined, they immediately cause a
stall which we would like to avoid.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>