anv: Add an anv_execbuf helper struct

anv: Add an anv_execbuf helper struct

This commit adds a little helper struct for storing everything we use to
build an execbuf2 call.  Since the add_bo function really has nothing to do
with a command buffer, it makes sense to break it out a bit.  This also
reduces some of the churn in the next commit.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Cc: "13.0" <mesa-stable@lists.freedesktop.org>