i965: Rename BRW_MESSAGE_TARGET_* to BRW_SFID_* and document them.

i965: Rename BRW_MESSAGE_TARGET_* to BRW_SFID_* and document them.

When reading the data port code, it was not clear to me what these
values meant, nor where I could find them in the documentation.
Especially since the latest BSpec and older PRMs document them in
radically different places...neither of which are near the descriptions
of individual messages.

Cite the documentation, and rename them to SFID to signify that these
are Shared Function IDs that one can read about in the GPU overview,
rather than arbitrary bitfields.  While we're add it, make them an enum.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>