mhi: core: pm lock not needed to read pm state

Acquiring of pm lock is necessary if data is going over the PCIe
link to avoid another thread from turning off the link. For
reading pm state, acquiring pm lock is not necessary so remove
grabbing the lock when entering process ctrl event thread.

CRs-Fixed: 2248650
Change-Id: I68b894f41cac5a4a4c58bc94a20eaef4d70bf311
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>