msm: ipc: Add single-threaded workqueue for PIL voting

IPC Router votes for the modem by default to ensure that the modem is
loaded so QMI services can be discovered. However, this causes a large
delay measured in seconds for the first client to bind to an IPC Router
socket. For clients that do not need the modem, this delay is
unnecessary.

Move the PIL loading to a single-threaded workqueue to load the modem
and maintain reference counting asynchronously.

Change-Id: I2a2a375fa4538442e5a16465935a07ba1d7437b8
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>
Signed-off-by: Steven Cahail <scahail@codeaurora.org>