msm: kgsl: Hold driver process mutex till sysfs entries deleted

sysfs entries deletion logic is not under the driver
process mutex lock, there was a Camera test case
where a single process was trying to call
kgsl_open/kgsl_release very rapidly from multiple
thread, sometimes sysfs entries remained valid
and kgsl_open was trying to create new sysfs
entries for same process id. This caused
WARNING in kernel logs "sysfs: cannot create
duplicate filename '/devices/virtual/kgsl/kgsl/proc/277'"
This change holds the driver process mutex
lock till sysfs entries are deleted.

Change-Id: I0f66314e704c2b53657961303f76e2bd4f34e5b5
Signed-off-by: Prakash Kamliya <pkamliya@codeaurora.org>