Fix concurrent access to IDR in kernel driver

The IDR instance must be protected against concurrent access and the
device mutex in the kernel driver should be used to protect it. However,
the device mutex is not locked when an inference is released or when the
mailbox is cleared which means the IDR is not protected in these
instances.

To resolve this, the missing mutex locks have been added and the
functions using the IDR instance have been updated to make it clearer
that the device mutex is expected to be locked when called.

Change-Id: Id0b314db556836c36663d6481806b7c113e55e5f
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>