[PATCH] Added get_num_groups

The get_num_groups function was missing for r600g. I did the same
thing as the other workitem functions.

The get_num_groups function was missing for r600g. I did the same
thing as the other workitem functions.

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>

I've pushed this patch. Thanks for writing it.

--Aaron