[PATCH] Added get_num_groups

Runtimes OpenCL
#1

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

#2

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>

#3

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

--Aaron