msm: pm: Ignore retention modes if regulator has disabled it

The regulator driver disables a retention mode if the processor is
running at voltage which cannot support the retention voltage. In these
scenarios, prevent retention mode from being selected instead of failing
after retention mode has been selected.

CRs-fixed: 606564
Change-Id: Id42b7fe1f9e861f88d3df82d38300d63221c4156
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>