ARM: OMAP2: PM/powerdomain: drop unnecessary pwrdm_wait_transition()

Drop an unnecessary pwrdm_wait_transition() from mach-omap2/pm.c -
it's called by the subsequent pwrdm_state_switch().

Also get rid of pwrdm_wait_transition() in the powerdomain code - there's
no longer any need to export this function.

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