ARM: OMAP3: PM: call pre/post transition per powerdomain

We only need to call the pre/post transtion methods when we know the
power state is changing.  First, split up the pre/post transition
calls to be per-powerdomain, and then make them conditional on whether
the power domain is actually changing states.

[notasas@gmail.com: backport to 3.2]
Reviewed-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>