i965: Attempt to un-muddle Gen6 data port message target defines.

i965: Attempt to un-muddle Gen6 data port message target defines.

These are documented on page 245 of IHD_OS_Vol4_Part2.pdf (the public
Sandybridge documentation/SEND instruction description).

Somebody had the bright idea to reuse gen4/5 defines labelled READ/WRITE
which just happened to be the same values as Render Cache/Sampler Cache.
It turns out that this field has nothing to do with READ/WRITE on
Sandybridge, but rather represents which data port to direct it to.

This was especially confusing in brw_set_dp_read_message, which
used "BRW_MESSAGE_TARGET_DATAPORT_WRITE."  In a read function.

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