intel/compiler: Handle bits 15:12 in brw_send_indirect_split_message()

intel/compiler: Handle bits 15:12 in brw_send_indirect_split_message()

Annoyingly, these bits exist in some extended message descriptors
(in particular render target writes), but they don't have any
corresponding bits in the ISA encoding.  So we can't use an immediate
and have to fall back to an indirect extended descriptor.

Thanks to Jason Ekstrand for reminding me that you can still set these
bits via an indirect descriptor, even if they don't exist in the ISA.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>