msm: Allow VPE driver to be configured directly.

Currently VPE hardware is being controlled through
the config node. Since it is converted into a V4L2
subdevice, we could control it directly from
userspace by issuing open, close and ioctl system
calls.
The ACK from VPE hardware shall be routed directly
to userspace by putting the ACK event on the v4l2
event queue.

Change-Id: I7f82a750a28732382627349da362dbee8e27b149
Signed-off-by: Kiran Kumar H N <hurlisal@codeaurora.org>