i965/fs: Define logical framebuffer write opcode.

i965/fs: Define logical framebuffer write opcode.

The logical variant is largely equivalent to the original opcode but
instead of taking a single payload source it expects its arguments
that make up the payload separately as individual sources, like:

 fb_write_logical null, color0, color1, src0_alpha,
                        src_depth, dst_depth, sample_mask, num_components

This patch defines the opcode and usual instruction boilerplate,
including a placeholder lowering function provided mainly as
self-documentation.

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