i965: Split out brw_<stage>_populate_key into their own functions

i965: Split out brw_<stage>_populate_key into their own functions

This commit splits portions of the existing brw_upload_vs_prog and
brw_upload_gs_prog function into new brw_vs_populate_key and
brw_gs_populate_key functions. This follows the same style as is
already present for all other stages, (see brw_wm_populate_key, etc.).

This commit is intended to have no functional change. It exists in
preparation for some upcoming code movement in preparation for the
shader cache.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>