i965: Factor out logic to build a send message instruction with indirect descriptor.

i965: Factor out logic to build a send message instruction with indirect descriptor.

This is going to be useful because the Gen7+ uniform and varying pull
constant, texturing, typed and untyped surface read, write, and atomic
generation code on the vec4 and fs back-end all require the same logic
to handle conditionally indirect surface indices.  In pseudocode:

|   if (surface.file == BRW_IMMEDIATE_VALUE) {
|      inst = brw_SEND(p, dst, payload);
|      set_descriptor_control_bits(inst, surface, ...);
|   } else {
|      inst = brw_OR(p, addr, surface, 0);
|      set_descriptor_control_bits(inst, ...);
|      inst = brw_SEND(p, dst, payload);
|      set_indirect_send_descriptor(inst, addr);
|   }

This patch abstracts out this frequently recurring pattern so we can
now write:

| inst = brw_send_indirect_message(p, sfid, dst, payload, surface)
| set_descriptor_control_bits(inst, ...);

without worrying about handling the immediate and indirect surface
index cases explicitly.

v2: Rebase.  Improve documentatation and commit message. (Topi)
    Preserve UW destination type cargo-cult. (Topi, Ken, Matt)

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>