i965/fs: Communicate the pull constant block read parameters through fs_regs.

i965/fs: Communicate the pull constant block read parameters through fs_regs.

I wanted to add the surface index as a variable value for UBO support,
and a reg seemed like the obvious way to go.  This exposes more of the
information to CSE, which we'll probably want to apply to pull
constant loads for UBOs eventually (you might access 4 floats in a
row, each of which would produce an oword block read of the same
block).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>