lib: Fix assert failure in ring stop code

lib: Fix assert failure in ring stop code

The assert in igt_set_stop_rings() is a bit bogus. Suppsoedly it was
meant to assure that the caller always restores the rings to running
state after stopping them. But in its current form it will trigger
any time someone calls the function and the rings were already stopped.
So you can't even restore the rings to running state w/o the assert
triggering.

Fix it up so that it'll ignore the current ring state when we're trying
to restore the rings back to running state.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>