gru: support contexts with zero dsrs or cbrs

Support alocation of GRU contexts that contain zero DSR or CBR resources.
Some instructions do not require DSR resources.  Contexts without CBR
resources are useful for diagnostics.

Signed-off-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>