gallium: add a new interface for pipe_context::launch_grid()

gallium: add a new interface for pipe_context::launch_grid()

This introduces pipe_grid_info which contains all information to
describe a launch_grid call. This will be used to implement indirect
compute in the same fashion as indirect draw.

Changes from v2:
 - correctly initialize pipe_grid_info for nv50/nvc0

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>