ASoC: Reduce the number of neigbours we mark dirty when updating power

If two widgets are not currently connected then there is no need to
propagate a power state change between them as we mark the affected
widgets when we change a connection. Similarly if a neighbour widget is
already in the state being set for the current widget then there is no
need to recheck.

On one system I tested this gave:

           Power    Path   Neighbour
Before:    114      1066   1327
After:     106      970    1186

which is an improvement, although relatively small.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>