intel/compiler: rework message descriptors for render targets

intel/compiler: rework message descriptors for render targets

Render target message descriptors are slightly different from the
dataport ones. In particular the msg_type field is on bits 14:17 for
RT while bits 14:18 for DP.

v2: Drop unused send_commit_msg field in brw_fb_write_desc() (Ken)

v3: Rebase on top renaming (Lionel)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Suggested-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7455>