i965/fs: Reimplement emit_single_fb_write() in terms of logical framebuffer writes.

i965/fs: Reimplement emit_single_fb_write() in terms of logical framebuffer writes.

The only non-trivial thing it still has to do is figure out where to
take the src/dst depth values from and predicate the instruction if
discard is in use.  The manual SIMD unrolling logic in the dual-source
case goes away because this is now handled transparently by the SIMD
lowering pass.

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