i965/fs: Implement lowering of logical framebuffer writes.

i965/fs: Implement lowering of logical framebuffer writes.

This does essentially the same thing as
fs_visitor::emit_single_fb_write(), with some slight differences:

 - We don't have to worry about exec_size and use_2nd_half anymore,
   16-wide sources have already been lowered to 8-wide thanks to the
   previous commit and the manual argument unzipping is no longer
   required.

 - The src/dst_depth and sample_mask values are now explicit sources
   of the instruction instead of being taken from the visitor state
   directly.  The same goes for the kill-pixel mask that will be
   passed to the instruction explicitly as predicate.

 - Everything is now done in static functions to improve
   encapsulation.

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