prog_to_nir: Use a variable for uniform data

prog_to_nir: Use a variable for uniform data

Previously, the prog_to_nir pass was directly generating uniform load/store
intrinsics.  This converts it to use a single giant "parameters" variable
and we now depend on lowering to get the uniform load/store intrinsics.
One advantage of this is that we now have one code-path after we do the
initial conversion into NIR.

No shader-db changes.

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>