gallium: Basic compute interface.

gallium: Basic compute interface.

Define an interface that exposes the minimal functionality required to
implement some of the popular compute APIs.  This commit adds entry
points to set the grid layout and other state required to keep track
of the usual address spaces employed in compute APIs, to bind a
compute program, and execute it on the device.

Reviewed-by: Marek Olšák <maraeo@gmail.com>