i965: Use full-length PIPE_CONTROL packets for workaround writes.

i965: Use full-length PIPE_CONTROL packets for workaround writes.

I believe that PIPE_CONTROL uses the length field to decide whether to
do 32-bit or 64-bit writes.  A length of 4 would do a 32-bit write,
while a length of 5 would do a 64-bit write.  (I haven't verified this,
though.)

For workaround writes, we don't care what value gets written, or how
much data.  We're only writing something because hardware bugs mandate
that do so.  So using a 64-bit write should be fine.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>