msm: kgsl: Use worker to put refcount on mem entry

If refcount on mem entry is the last refcount, we will call
kgsl_mem_entry_destroy and detach it from process list. If
there is no refcount on the process private as well then we
will call kgsl_destroy_process_private to cleanup. During
cleanup, we will try to remove the same sysfs node which is
in use by the current thread and this situation will end up in
a deadlock.
To fix this problem, use a worker to put the refcount on mem
entry.

Change-Id: I69a77c4059489c95de2ca205701571daac4bdc8a
Signed-off-by: Pankaj Gupta <gpankaj@codeaurora.org>