drm/msm: use new mutex for protecting address space lists

Added a new list_lock mutex to protect access to the active_list and
clients lists as opposed to relying on the global dev->struct_mutex.
This is needed since in porting aspace from 4.9 to 4.14, the device
struct_mutex is not always used by the upper layers which access
these lists.

Change-Id: I9d3714de2a8be6021a0554e4fe6a4f4dcd3f71d7
Signed-off-by: Steve Cohen <cohens@codeaurora.org>