anv: Rework push constant handling

anv: Rework push constant handling

This substantially reworks both the state setup side of push constant
handling and the pipeline compile side.  The fundamental change here is
that we're no longer respecting the prog_data::param array and instead
are just instructing the back-end compiler to leave the array alone.
This makes the state setup side substantially simpler because we can now
just memcpy the whole block of push constants and don't have to
upload one DWORD at a time.

This also means that we can compute the full push constant layout
up-front and just trust the back-end compiler to not mess with it.
Maybe one day we'll decide that the back-end compiler can do useful
things there again but for now, this is functionally no different from
what we had before this commit and makes the NIR handling cleaner.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>