msm: kgsl: Remove debugfs directory inside lock

In kgsl_process_private_close, debugfs directory is removed after mutex
is unlocked. Considering this, a race can be created between
kgsl_process_private_close and kgsl_process_init_debugfs. Fix it by
moving debugfs directory removal inside lock.

Change-Id: Ida65ab8a3825d8c695c56556860495cce853117c
Signed-off-by: Kamal Agrawal <kamaagra@codeaurora.org>