gru: check context state on reload

Check whether the gru state being loaded into a gru is from a new context
or a previously unloaded context.  If new, simply zero out the hardware
context; if unloaded and valid, reload the old state.

This change is primarily for reloading kernel contexts where the previous
is not required to be saved.

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>