msm: adsprpc: do pm stay awake vote in rpmsg callback

Vote with PM to keep the CPU awake in the rpmsg interrupt callback
itself, instead of voting from the fastrpc thread after it is
woken up and scheduled, to handle race between actual remote
subsystem response and context interruption because of CPU going
into suspend.

Change-Id: I289f4307aacc895aa73c51360f7cf83131a72a13
Acked-by: Thyagarajan Venkatanarayanan <venkatan@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>