Delay for a usec while spinning waiting for ring buffer space.

This means the loop will wait up to ~10ms for ring buffer space to become
available, rather than just however long it takes to check the space 10000
times.  This matches other drivers' behavior when waiting for ring buffer/fifo
space.

shared-core/i915_dma.c

@@ -63,6 +63,7 @@ int i915_wait_ring(drm_device_t * dev, int n, const char *caller)
 			i = 0;
 
 		last_head = ring->head;
+		DRM_UDELAY(1);
 	}
 
 	return DRM_ERR(EBUSY);