ARM: OMAP3xxx: CPUIdle: simplify the PER next-state code

The OMAP3xxx CPUIdle driver contains some code to place a lower bound
on the PER powerdomain's power state.  Convert this code to a data-driven
implementation to remove branches and to improve readability.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>