i965/fs: Migrate FS framebuffer writes to the IR builder.

i965/fs: Migrate FS framebuffer writes to the IR builder.

The explicit call to fs_builder::group() in emit_single_fb_write() is
required by the builder (otherwise the assertion in fs_builder::emit()
would fail) because the subsequent LOAD_PAYLOAD and FB_WRITE
instructions are in some cases emitted with a non-native execution
width.  The previous code would always use the channel enables for the
first quarter, which is dubious but probably worked in practice
because FB writes are never emitted inside non-uniform control flow
and we don't pass the kill-pixel mask via predication in the cases
where we have to fall-back to SIMD8 writes.

Reviewed-by: Matt Turner <mattst88@gmail.com>