i965: Replace fs_reg(fs_visitor, type) with fs_visitor::vgrf(type).

i965: Replace fs_reg(fs_visitor, type) with fs_visitor::vgrf(type).

I dislike how fs_reg has a constructor that knows about fs_visitor.
Apart from that, it stands alone, with no need to interact with the
rest of the compiler.  Which is sensible - a class that represents
a register should do just that.  Allocating virtual register numbers
should be left up to the compiler (fs_visitor).

This patch replaces the constructor with a new fs_visitor::vgrf method,
eliminating fs_reg's dependency on fs_visitor.  It ends up being no
more code.

v2: Rebase from May 2014 -> January 2015.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>