i965: Add a function to disassemble an instruction from the 4 dwords.

i965: Add a function to disassemble an instruction from the 4 dwords.

I used this a while back when debugging GPU hangs, and it seems like it
could be useful, so I figured I'd add it so people can use it in the
debugger.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>