intel/bdw/aub: Update AUB trace block writes for 48-bit addressing.

Since our aub file dumping's GTT handling is totally fake, we always put
everything in the low 4GB anyway and shouldn't ever need to set
AddressHigh to anything other than 0.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
[ben: slight commit message change]
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>