r600g: Add start_compute_cs atom to struct r600_context

r600g: Add start_compute_cs atom to struct r600_context

The start_compute_cs atom initializes some config and context registers
to the values needed for running compute shaders.  When a compute shader
is dispatched, this atom is emitted after the start_cs_cmd atom, which
initializes registers that are common to both 3D and compute.

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